Replace MR access checks with use of MergeRequestsFinder

Split from !2024 to partially solve https://gitlab.com/gitlab-org/gitlab-ce/issues/23867

⚠ - Potentially untested
💣 - No test coverage
🚥 - Test coverage of some sort exists (a test failed when error raised)
🚦 - Test coverage of return value (a test failed when nil used)
✅ - Permissions check tested

- [x] 💣  app/finders/notes_finder.rb:17
- [x] ⚠  app/views/layouts/nav/_project.html.haml:80 [`.count`]
- [x] 💣  app/controllers/concerns/creates_commit.rb:84
- [x] 🚥  app/controllers/projects/commits_controller.rb:24
- [x] 🚥  app/controllers/projects/compare_controller.rb:56
- [x] 🚦  app/controllers/projects/discussions_controller.rb:29
- [x] ✅  app/controllers/projects/todos_controller.rb:27
- [x] 🚦  app/models/commit.rb:268
- [x] ✅ lib/gitlab/search_results.rb:71

- [x] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#d1c10892daedb4d4dd3d4b12b6d071091eea83df_267_266 Memoize ` merged_merge_request(current_user)`
- [x] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#d1c10892daedb4d4dd3d4b12b6d071091eea83df_248_247 Expected side effect for `merged_merge_request!`, consider `skip_authorization: true`.
- [x] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#d1c10892daedb4d4dd3d4b12b6d071091eea83df_269_269 Scary use  of unchecked `merged_merge_request?`

See merge request !2033

So we have raw_diffs too

This object will manage Gitlab::Git::Compare instances

Instead calling diff_collection.count use diff_collection.size which is cache on the diff_collection

Introducing the concept of SafeDiffs which relates 
diffs with UI highlighting.

Closes #19573

1. Move render method to a concern, not a helper.
2. Let DiffHelper#diff_options automatically add the path option.
3. Move more instance var definitions to before filters.

When rendering a list of diff files, skip those where the diff is over
10 KB and provide an endpoint to render individually instead.

Extract diff line data attributes to helper methods

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

* Use commit objects instead of IDs when generating diffs
* Use proper references when generating MR's source and target
* Update broken specs

Now we apply syntax highlighting to the whole old and new files.
This basically help us to highlight adequately multiline content.

fix this issue(https://gitlab.com/gitlab-org/gitlab-ce/issues/1393).

Add ignore whitespace optoin to Commits Compare view

Closes #2593

This reverts commit 5daf44b7, reversing
changes made to 2f706fbd.

This reverts commit 957e849f, reversing
changes made to 6b9dbe9f.
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Closes #1399
Closes https://github.com/gitlabhq/gitlabhq/issues/9105

Signed-off-by: NJeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Make the following changes to deal with new behavior in Rails 4.1.2:

* Use nested resources to avoid slashes in arguments to path helpers.

with existing method_missing. Pattern already used extensively,
so let's be consistent and use it everywhere.

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