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

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>

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

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

Reuse HTML Pipeline object in Markdown parser

The patches reduces pressure on GC and reduces markdown processing time

See merge request !905

The patches reduces pressure on GC and reduces markdown processing time