Given the branch name 'mep-mep' with an iid being 1,
the current way, master's way, would yield a branch name of
1-mep-mep. The problem for larger projects however would be that
a developer might forget what iid the issue was.

When this developer would try to tab complete it would:
- Or result in 20+ branches possibly
- Or start with the wrong digit, try again with digit++
  - Would see 20 branches, repeat

Thus the obvious way of solving this is letting the dev tab complete
on the issue title, which is easier to remember.

We have to ensure source_sha and target_sha are not nil before calling
Gitlab::Git::Commit.between.

Since we're not actually returning a todo reference.

This builds on the changes introduced in
https://gitlab.com/gitlab-org/gitlab_git/merge_requests/72 and results
in merge requests with large diffs (e.g. due to them containing minified
CSS) loading much faster.

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

Spinach was right, I was a fool..

`MergeRequest#source_sha` is expected to return the sha of the source
branch last commit.

But when a 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 un-noticed before – but now that !2217 has been merged,
it makes `Gitlab::Git::Commit.between` raise an
"Unexpected nil argument" exception.

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

The feature spec now also tests the absence of the new branch button