Closes https://gitlab.com/gitlab-org/gitaly/issues/326

Batching commits for performance improvements, might lead to empty
batches being used. This isn't the case yet, but to guard against this
in future cases, a guard clause is added.

Prior to this change, this was done through unicorn. In theory this
could time out. Workhorse has been sending these raw patches and diffs
for a long time and is stable in doing so.

Added bonus is the fact that `Commit#to_patch` can be removed.

`Commit#to_diff` too, which closes
https://gitlab.com/gitlab-org/gitaly/issues/324

Closes https://gitlab.com/gitlab-org/gitaly/issues/1196

Closes gitaly#1046

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

- Added tests for Git::Commit.shas_with_signatures

Replaces all the explicit include metadata syntax in the specs (tag:
true) into the implicit one (:tag).
Added a cop to prevent future errors and handle autocorrection.

Closes gitaly#396