Fix MergeRequest#source_sha when there is no diff

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14170

### Overview

This MR fixes an unhandled Exception when visiting the page of an open Merge Request  without diff.

### Description

`MergeRequest#source_sha` is expected to return the sha of the source branch last commit. But when an open Merge Request has no diff (e.g. all commits have already been merged to the target branch), `merge_request.source_sha` incorrectly returns `nil`.

This was without consequences before – but since !2217 was merged (a few days ago), it makes `Gitlab::Git::Commit.between` raise an "Unexpected nil argument" exception. This can be reproduced when visiting the http://localhost:3000/gitlab-org/gitlab-test/merge_requests/2 page on a fresh local Gitlab setup.

This MR fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available. I also added tests.

@DouweM I believe you wrote most of this code in the first place ; does this looks correct to you, or is there a better way to resolve this issue maybe?

See merge request !3135

Removed diverging commit count calculation from Repository#build_cache

Using a repository with 1000 branches the old `Repository#build_cache` method would take around 180 seconds to complete. Without calculating the diverging commit counts this method "only" takes around 60 seconds. 

See commit 28cc2413eb5ddf920ce0b5eed803121f8b884754 for more details. This fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14058

cc @rspeicher 

See merge request !3274

Add avatar to issue and MR pages header

closes #14395
# For Issues

### Before
![Screen_Shot_2016-03-18_at_4.27.25_PM](/uploads/b097c8e304c9737e2f46ee25667829bf/Screen_Shot_2016-03-18_at_4.27.25_PM.png)

### After
![Screen_Shot_2016-03-18_at_4.22.10_PM](/uploads/f7d2b12c9455950c2e660b33e27c1827/Screen_Shot_2016-03-18_at_4.22.10_PM.png)

# For Merge Request

### Before
![Screen_Shot_2016-03-18_at_4.27.09_PM](/uploads/50c7b6ee346f749f448647004ae6031b/Screen_Shot_2016-03-18_at_4.27.09_PM.png)

### After
![Screen_Shot_2016-03-18_at_4.24.17_PM](/uploads/abd37ccf030e0b6d10521a6937206de6/Screen_Shot_2016-03-18_at_4.24.17_PM.png)

See merge request !3301

Fixes issue with any milestone value in dropdown not being selectable

Closes #14293

See merge request !3234

Fixes issue with filter label missing on labels & milestones

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14407

See merge request !3297

Changing filter dropdowns shows loading

Instead of doing a full refresh of the page - i've modified the filterResults method on the Issues object to work for this form as well

![issues](/uploads/3335f09f7df88b6d419de7fd3d6857d2/issues.gif)

Closes #14359

See merge request !3237

Add confidential issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3678

More information: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/227

See merge request !3282

Prevent a 500 when a repository's root_ref is nil

Closes #14259

See merge request !3278

Improved issue sidebar

Updated the spacing throughout the sidebar so that it fits better on smaller screens. Also adds a more participants button to show any other participants after the first row.

Closes #13353

See merge request !3097

Dashboard labels

Previously because there were no JSON endpoint for labels or milestones
it was fetching HTML and parsing that. This is wrong.

It now fetches from a JSON endpoint.

This also fixes the dashboard/issues page not loading labels or
milestones as the path used to be only for a single project. So instead
I have created a endpoint for labels & milestones JSON on dashboard.

See merge request !3253

Fixes issue with close button not working on MR

Closes #14383

See merge request !3295

Add note in upgrade guide to choose the correct version tag

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3448

@rspeicher I'm assigning to you since as a release manager you should know better. Let's keep it in this guide for now and then start copying it in the future ones.

See merge request !2284

Show a notice for diffs that are empty/too large

This builds upon https://gitlab.com/gitlab-org/gitlab_git/merge_requests/72 but will fail until said MR has been merged.

Fixes gitlab-org/gitlab-ce#13912, gitlab-org/gitlab-ce#13644, gitlab-org/gitlab-ce#13557, gitlab-org/gitlab-ce#13267, gitlab-org/gitlab-ce#3906

See merge request !3185

[ci skip]

Add side padding to content-list if inside panel
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fixes #14367, #14349

See merge request !3273

Use relative links in `doc`, fix broken generated HTML links

Links like (doc/web_hooks/web_hooks.md) work in the GitLab source code
web interface, but the HTML generator produces broken links in the `doc`
subdirectories:  
http://doc.gitlab.com/ce/hooks/doc/web_hooks/web_hooks.html  
instead of the right one  
http://doc.gitlab.com/ce/web_hooks/web_hooks.html  
in  
http://doc.gitlab.com/ce/hooks/custom_hooks.html.

Fixes #14338 


See merge request !3261

Remove CI duplication.



See merge request !3290

Improve award emoji test reliability by checking that the emoji is displayed before the search field is focused

See merge request !3288

Check push permissions only when pushing directly to target branch.

Fixes #14374 

See merge request !3280

Fix bug causing dropdown to get cut off in project page

closes #14336 

### Before
![Screen_Shot_2016-03-16_at_11.30.40_PM](/uploads/9f8dd10f0627cdfbd49d7127b233b105/Screen_Shot_2016-03-16_at_11.30.40_PM.png)

![Screen_Shot_2016-03-16_at_11.30.53_PM](/uploads/6dfe64d68867d158a11be7e74cbd1520/Screen_Shot_2016-03-16_at_11.30.53_PM.png)


### After
![Screen_Shot_2016-03-16_at_11.30.14_PM](/uploads/2b87cc17522e8a425a97a78b36982873/Screen_Shot_2016-03-16_at_11.30.14_PM.png)

See merge request !3257

Fix inconsistency in the header of issue and merge request pages

closes #13550 

# For Issue Page

### Before
![Screen_Shot_2016-03-17_at_8.46.59_PM](/uploads/ede7f2b98395362ae87e6258cc10520a/Screen_Shot_2016-03-17_at_8.46.59_PM.png)

### After
![Screen_Shot_2016-03-17_at_8.48.56_PM](/uploads/da7adf17cb397ddc0677a6a5e8033b8e/Screen_Shot_2016-03-17_at_8.48.56_PM.png)

# For Merge request Page

### Before
![Screen_Shot_2016-03-17_at_8.45.45_PM](/uploads/10137827c05c8166a0aa429687e50d1b/Screen_Shot_2016-03-17_at_8.45.45_PM.png)

### After
![Screen_Shot_2016-03-17_at_8.45.01_PM](/uploads/92151c03cfe29b71090a1e0ad31abd6e/Screen_Shot_2016-03-17_at_8.45.01_PM.png)


See merge request !3279

Fixed alignment of download dropdown

With the new dropdown CSS - some dropdowns might look a bit odd

## Previously

![Screen_Shot_2016-03-16_at_11.46.04](/uploads/c1a8fc95ed4410de35d7bd5caedf28f5/Screen_Shot_2016-03-16_at_11.46.04.png)

## Now

![Screen_Shot_2016-03-16_at_11.45.26](/uploads/998637fe0872b2e826a8c17d183499dc/Screen_Shot_2016-03-16_at_11.45.26.png)

See merge request !3245

Fixed filter spacing

Closes #14350

![Screen_Shot_2016-03-17_at_10.21.33](/uploads/5b8f39134cc90ce1dd6565f665a39838/Screen_Shot_2016-03-17_at_10.21.33.png)

See merge request !3267

Bump Git version requirement to 2.7.4

[ci skip]

See merge request !3281

Edited timeago text on comments

Unified the 'edited text' to be the same in descriptions and comments

Closes #5538 

See merge request !3063

Revert color for links inside gray areas

**Before**

![Screen_Shot_2016-03-17_at_12.23.17_PM](/uploads/55e518721d83c3bfc42edba3e97ec44d/Screen_Shot_2016-03-17_at_12.23.17_PM.png)

**After**

![Screen_Shot_2016-03-17_at_12.23.03_PM](/uploads/08fbf8e9139e4eadf191102cffd29617/Screen_Shot_2016-03-17_at_12.23.03_PM.png)

Reverts !3147

See merge request !3276

Hide builds from Project's settings when the feature is disabled

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/13944

See merge request !3220

Cache project avatars stored in Git

Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/14363

See merge request !3272

The avatar logic has been moved from Project to Repository as this makes
caching easier. The logic itself in turn has been changed so that the
logo file names are cached in Redis. This cache is flushed upon pushing
a commit but _only_ if:

1. The commit was pushed to the default branch
2. The commit actually changes any of the logo files

If no branch or commit is given the cache is flushed anyway, this
ensures that calling Repository#expire_cache without any arguments still
flushes the avatar cache (e.g. this is used when removing a project).

Fixes gitlab-org/gitlab-ce#14363

Pre-selects values on issue and merge request dropdown

Closes #14290

See merge request !3233

Adjusted behavior so canceled builds tagged as allowed to fail do not fail build

## What does this MR do?
This diff changes the 'Allowed to fail' flag to also ignore canceled builds, whereas before, canceled builds could fail the suite even if they were marked as 'Allowed to fail'

dupe of !3258 to force a build

## Are there points in the code the reviewer needs to double check?
no

## Why was this MR needed?
Unexpected behavior as a user

## What are the relevant issue numbers?

## Screenshots (if relevant)
![whatisgoingon](/uploads/fcd7c8f3d8454bc730d7fd41eff59b31/whatisgoingon.png)

See merge request !3271

Merge duplicate cache entry in CI docs



See merge request !3208