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

Closes gitlab-org/gitaly#348

Closes https://gitlab.com/gitlab-org/gitaly/issues/382

Closes https://gitlab.com/gitlab-org/gitaly/issues/1082

Closes https://gitlab.com/gitlab-org/gitaly/issues/865
Closes https://gitlab.com/gitlab-org/gitaly/issues/1018

Migration: https://gitlab.com/gitlab-org/gitaly/issues/1026

Conflicts:
	lib/gitlab/git/repository.rb

Closes https://gitlab.com/gitlab-org/gitaly/issues/330

Closes https://gitlab.com/gitlab-org/gitaly/issues/217

Closes https://gitlab.com/gitlab-org/gitaly/issues/217

Closes: https://gitlab.com/gitlab-org/gitaly/issues/389

Closes https://gitlab.com/gitlab-org/gitaly/issues/390

Closes: https://gitlab.com/gitlab-org/gitaly/issues/220

Migration: https://gitlab.com/gitlab-org/gitaly/issues/220

Migration: https://gitlab.com/gitlab-org/gitaly/issues/220

Initially expected this to be a full blown migration, but that wasn't
the case as we could fetch to a reference that would be deleted after
the instanciation of the Diff object.

This is a racy RPC, as #delete_refs is called and garbage collections
runs right than, the commit can't be found later.

Closes https://gitlab.com/gitlab-org/gitaly/issues/771

Closes #45743

OPT_OUT status has been removed, and alternative implementation removed.
Also checks if the repository exists before executing the checksum RPC
to guard against NotFound errors.

Closes gitlab-org/gitaly#1105