Handle nil commits in Gitlab::PushDataBuilder.build

Closes #13469

See merge request !2825

Expire caches after forking/importing a repository

cc @dblessing @DouweM 

Related issue: gitlab-org/gitlab-ce#13505

See merge request !2838

Fixes for showing images

- Render scrubbed SVG's inline, not via /raw/
- Use /raw/ requests on image diffs

Both of these are needed because of no longer eager loading blobs.

This a follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2830

Also see https://gitlab.com/gitlab-org/gitlab-ce/issues/13350

See merge request !2837

Fix a bug preventing from doing subsequent edits in any Issuable sidebar

Fixes #13517. Fixes #12572.

### Issue

![impossible-subsequent-issue-edits-in-sidebar](/uploads/bd45c0effb46848641932d9cd38d3fe2/impossible-subsequent-issue-edits-in-sidebar.gif)

---

### Merge request

![impossible-subsequent-mr-edits-in-sidebar](/uploads/b4e1e3e93df4ac0a3d962df8b6139de9/impossible-subsequent-mr-edits-in-sidebar.gif)

/cc @jschatz1 @sytses 

See merge request !2848

Fix 500 error when comparing by tags

_Originally opened at !2787 by @klowner._

- - -

Fixes #13171.
Fixes #13370.

See merge request !2849

Fix double scrollbar issues

Sidebar double scrollbar issue fix.

![Screen_Shot_2016-02-16_at_9.22.49_PM](/uploads/cb950eda08ed5d3b4a6c6979c977e029/Screen_Shot_2016-02-16_at_9.22.49_PM.png)

See merge request !2851

Prevents floated button group on issues view

Issue #13359 

![screenshot](/uploads/962ac4f3f1f3d6ef87f7152c09090801/screenshot.jpg)

See merge request !2826

This ensures that _all_ caches (including any caches normally only
flushed under certain conditions) are flushed whenever a project is
removed. Because cache keys are based on project namespaces (excluding
IDs) not doing so could result in a newly created project re-using old
caches (if the project was re-created using the same name).

This ensures the caches for Repository#empty? and
Repository#has_visible_content? are flushed after a repository has been
imported or forked.

Fixes gitlab-org/gitlab-ce#13505

Updated the allocations Gem to 1.0.4

Sometimes the output of Allocations.to_hash may contain garbage. See
https://gitlab.com/gitlab-org/allocations/commit/15d3a39f51392013dcf5f88dd21fab31201350ce
for more information.

See merge request !2816

[Award Emoji]: sporadic spec fix



See merge request !2833

fix table in doc/ci/variables/README.md

The table was missing a column separator, causing GitLab to render the
markdown incorrectly.

[ci skip]

See merge request !2840

Fix Redis installation guide



See merge request !2839

[ci skip]

Sidebar fixes

Fixes #13215 

* when editing a dropdown, but not changing anything you end up in an ugly state
* the hover-over state for the subscribe and other buttons are veeery subtle
* the collapse button needs some sort of hover-over state
* the reference gets shortened. It always got shortened. Not an issue with this in particular.
* the minimized sidebar is not functional. The copy button doesn't do anything
* looks all funky in safari (not for me)

See merge request !2736

Add new data to project in push, issue, merge-request and note webhooks data

_Originally opened at !2738 by @bugagazavr._

- - -

**What does this MR do?**

Adds more data to hooks, makes repository data in all hook more dry ( use one hook_attrs method for repository data in all hooks )

**Are there points in the code the reviewer needs to double check?**

1. https://gitlab.com/bugagazavr/gitlab-ce/blob/extend-hooks/app/models/project.rb#L738
2. https://gitlab.com/bugagazavr/gitlab-ce/blob/extend-hooks/app/models/concerns/issuable.rb#L132
3. https://gitlab.com/bugagazavr/gitlab-ce/blob/extend-hooks/lib/gitlab/push_data_builder.rb#L52
4. https://gitlab.com/bugagazavr/gitlab-ce/blob/extend-hooks/lib/gitlab/note_data_builder.rb#L56

**Why was this MR needed?**

More information about user for push events, and more data about repositiry ( project )

**What are the relevant issue numbers?**

No

**Screenshots (if relevant)**

No

See merge request !2788

Fix the "x of y" displayed at the top of Issuables' sidebar

1. We now display the index of the current issuable among all its project's
issuables, of the same type and with the same state.
1. Also, refactored a bit the Issuable helpers into a new `IssuablesHelper`
module.
1. Added acceptance specs for the sidebar counter.

Note: I didn't add a CHANGELOG item since it's a bug fix for an unreleased version.

Fixes #13356.

See merge request !2818

Cleaned up Repository#initialize

The "default_branch" argument is never used and the "project" argument
isn't optional.



See merge request !2832

Eager-load image blob data in diffs

Since gitlab_git 8.0, blob data are lazy-loaded so we have to call
`blob.load_all_data!(repo)` to eager-load them.

Fixes #13458.

/cc @jacobvosmaer @dblessing @rspeicher 

### New image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.18](/uploads/01c2afa2d25f3249b27744154ab2069b/Screen_Shot_2016-02-16_at_11.28.18.png)

#### After
![Screen_Shot_2016-02-16_at_11.30.56](/uploads/5ef72f717f22dfd0c1efa9e6d8ed926f/Screen_Shot_2016-02-16_at_11.30.56.png)

### Replaced image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.20](/uploads/39bb61851608643a86d9d7616139fc6c/Screen_Shot_2016-02-16_at_11.28.20.png)

#### After
![Screen_Shot_2016-02-16_at_11.30.59](/uploads/27e754fe8b57a838ac695bace27618c3/Screen_Shot_2016-02-16_at_11.30.59.png)

### Deleted image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.22](/uploads/3648dcc2c36bc644556080e203a29fca/Screen_Shot_2016-02-16_at_11.28.22.png)

#### After
![Screen_Shot_2016-02-16_at_11.31.01](/uploads/2585915d994bb6e245dd8c8553730bbf/Screen_Shot_2016-02-16_at_11.31.01.png)

### Image diff in merge request diff view

#### Before
![Screen_Shot_2016-02-16_at_11.28.37](/uploads/a7963f7139fc91975ca40568570ce4f0/Screen_Shot_2016-02-16_at_11.28.37.png)

#### After
![Screen_Shot_2016-02-16_at_11.31.04](/uploads/6773dbf3b870f082f0262febc9722d20/Screen_Shot_2016-02-16_at_11.31.04.png)

See merge request !2830

Fix cross reference notes  on forks

Updates `cross_reference_exists?` to match on commit only.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/5849

See merge request !2731

Ensure Commit#show responds 404 instead of 500 when given an invalid ID

Closes #13467

See merge request !2823

Properly render the `errors/git_not_found` page



See merge request !2824

The "default_branch" argument is never used and the "project" argument
isn't optional.