Closes gitaly#929

Closes #42249

Closes gitaly#907

Closes gitaly#851

Migration is done through a small refactoring, which makes us call
endpoins which are performing the same actions for namespaces.
Tests are added to ensure only the project is removed that should be
removed.

Closes gitlab-org/gitaly#873

The previous implementation iterated across the entire patch set
to determine the number of lines added, deleted, and changed. Rugged
has a native method `Rugged::Diff#stat` that does this already,
which appears to be a little faster and require less RAM than doing
this ourselves.

Improves performance in #41524

Closes gitaly#825

Given the priorities shifted for the Gitaly team, this endpoint does not
get a dedicated endpoint yet. To make it work in a cloud native
environment the request needs to go to Gitaly, not rugged. This is
achieved by rerouting to the generic TreeEntry endpoint.

By importing this Ruby code into gitlab-rails (and gitaly-ruby), we avoid
200ms of startup time for each gitlab_projects subprocess we are eliminating.

By not having a gitlab_projects subprocess between gitlab-rails / sidekiq and
any git subprocesses (e.g. for fork_project, fetch_remote, etc, calls), we can
also manage these git processes more cleanly, and avoid sending SIGKILL to them

Moving the check out of the general requests, makes sure we don't have
any slowdown in the regular requests.

To keep the process performing this checks small, the check is still
performed inside a unicorn. But that is called from a process running
on the same server.

Because the checks are now done outside normal request, we can have a
simpler failure strategy:

The check is now performed in the background every
`circuitbreaker_check_interval`. Failures are logged in redis. The
failures are reset when the check succeeds. Per check we will try
`circuitbreaker_access_retries` times within
`circuitbreaker_storage_timeout` seconds.

When the number of failures exceeds
`circuitbreaker_failure_count_threshold`, we will block access to the
storage.

After `failure_reset_time` of no checks, we will clear the stored
failures. This could happen when the process that performs the checks
is not running.

This also allows us to simplify the naming since we can make some
fetching methods private.

- Added tests for Git::Commit.shas_with_signatures

That way we don't need the to scan the entire keyspace to get all
known keys

This was failing regularly with an Error 500 when the API branches endpoint
was used.

Closes #40615

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.