Closes https://gitlab.com/gitlab-org/gitaly/issues/817

Closes https://gitlab.com/gitlab-org/gitaly/issues/1236

Closes https://gitlab.com/gitlab-org/gitaly/issues/354

We request Gitaly in a N+1 manner to build discussion diffs. Once the diffs are from different revisions, it's hard to make a single request to the service in order to build the whole response.
With this change we solve this problem and simplify a lot fetching this piece of info.

For easier overriding methods.

A repo does not need pushes before it will be repo-checked. There are
other sources where commits can come from (e.g. import or webIDE).

In the test I made the method of breaking a repo more robust. And
added a few test examples.

See also gitlab-org/gitlab-ce#45046.

In case the repository checks are disabled, do not send the
mails. Otherwise it might send the same failures over and over again.