Refactor can_be_merged logic for merge request

Some refactoring after !918

cc @DouweM @rspeicher
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !919

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Merge branches inside one repository using rugged instead of satellites
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

cc @rspeicher @DouweM

See merge request !918

Extract the longest-matching ref from a commit path when multiple matches occur

### What does this MR do?

This MR extracts the longest-matching ref from a commit path. In cases when there are multiple refs that prefix the path, the ref name is ambiguous. Using the heuristic that the longest-matching ref seems like a sensible default.

### Why was this MR needed?

Suppose there is a branch named `release/app` and a tag named `release/app/v1.0.0`. Suppose `README.md` exists in the root directory.

Let's suppose the path passed in is `release/app/v1.0.0/README.md`. There are two possible ways to interpret the ref and path:

1. ref = `release/app`, path = `v1.0.0/README.md`
2. ref = `release/app/v1.0.0`, path = `README.md`

The crux of the issue is that there is ambiguity which one is correct; both could be real possibilities. In the current implementation of `extract_ref`, GitLab gets confused and tries neither: it uses ref = `release` and path = `app/v1.0.0/README.md`. Since the file does not exist, it returns 404.

### What are the relevant issue numbers?

Closes #1839

See merge request !859

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Closes #1839

Fix GitLab CI forking

I created MR here because gitlab.com is unreachable.

See merge request !1884

UX improvements

* make atom feed on commits page smaller so it capture less attention
* open big image 400px instead of default 40px when click on user avatar

See merge request !910

Fix consistency issue in sidebars of Project and Group Settings.

Fixes #2277

![before_after_group_settings_nav](https://dev.gitlab.org/gitlab/gitlabhq/uploads/370c701d0eff8f1f8e44cf954c75f2bf/before_after_group_settings_nav.png)

See merge request !1871

Repository#blob_at should return nil for 00000000... sha
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !916

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Reporter role can manage issue tracker now
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !908

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

added note about Devise

Added note about the authentication gem used in GitLab

Because of question in https://groups.google.com/forum/#!topic/gitlabhq/G2rToah19-Q

See merge request !912

added new doc "command line commands"

Added new doc to GitLab Basics

See merge request !1880

Performance improvements after caching html fragments

* re-use event cache with other users
* cache commit fragment
* cache issue fragment in list
* cache comment parsed with markdown

See merge request !911

Fix ApplicationHelper specs

On current `master` if you run these specs by themselves, they fail. I have no idea why they work when run with other specs, but now they work all the time.

- Use the `helper` object, as per RSpec 3 standards
- Use `assign` to assign instance variables that helpers expect
- Add `StubConfiguration` support module

See merge request !906

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Bump Redcarpet to 3.3.2

Closes #1911

See merge request !913

Closes #1911

There were several specs that were failing when run by themselves.

- Use the `helper` object, as per RSpec 3 standards
- Use `assign` to assign instance variables that helpers expect
- Add `StubConfiguration` support module

Just for consistency with our Capybara, DatabaseCleaner, WebMock, etc.
setups.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>