This old migration used Rugged to find a commit, while Gitaly is the
prefered way now. By migrating this to Gitaly, Gitaly is now a required
running component for this migration.

Part of https://gitlab.com/gitlab-org/gitaly/issues/1106

After trying to remove the whole method in
8f69014af2902d8d53fe931268bec60f6858f160, this is a more gentle
approach to the method. :)

Prior to this change, new commit detection wasn't implemented in Gitaly,
this was done through: https://gitlab.com/gitlab-org/gitaly/merge_requests/779

As the new implemented got moved around a bit, the whole RevList class
got removed.

Part of https://gitlab.com/gitlab-org/gitaly/issues/1233

This reverts merge request !20339

Prior to this change, most the commits counted were done through Gitaly.
This removes the last point where this wasn't the case.

This makes the `rugged_count_commits` method obsolete, with its tests.

Closes https://gitlab.com/gitlab-org/gitaly/issues/315

This prepares this code for migration to Gitaly.

This specific one isn't used on most machines, therefor low risk.

Closes https://gitlab.com/gitlab-org/gitaly/issues/944

Closes https://gitlab.com/gitlab-org/gitaly/issues/879

Closes https://gitlab.com/gitlab-org/gitaly/issues/308

Closes https://gitlab.com/gitlab-org/gitaly/issues/393

Has never been in OPT_OUT, dispite being ages old. The time where
feature flags are still set by env vars.

Closes https://gitlab.com/gitlab-org/gitaly/issues/221

Moved to the front on the line through: https://gitlab.com/gitlab-org/gitlab-ce/issues/47893

Closes: https://gitlab.com/gitlab-org/gitlab-ce/issues/47893
Closes: https://gitlab.com/gitlab-org/gitaly/issues/327

Closes https://gitlab.com/gitlab-org/gitaly/issues/1138

This now can happen because can_be_merged? is called
during MR merge_status transition to cannot_be_merged.
It is possible branch_name is invalid.

Closes https://gitlab.com/gitlab-org/gitaly/issues/889