The execution path wasn't clear from the error, but either way this is a
fix.

Closes gitlab-org/gitaly#1115

Acceptence testing was successfull in
[#844](https://gitlab.com/gitlab-org/gitaly/issues/844)

Part of gitlab-org/gitaly#670

Part of gitlab-org/gitaly#324

Closes gitaly#1046

The tracking issue if this causes problems is
https://gitlab.com/gitlab-org/gitaly/issues/1028

		    #	modified:   lib/gitlab/git/commit.rb

The refs hash is used to determine what branches and tags have a commit
as head in the network graph. The previous implementation depended on
Rugged#references. The problem with this implementation was that it
depended on rugged, but also that it iterated over all references and
thus loading more data than needed if for example the project uses CI/CD
environments, Pipelines, or Merge Requests.

Given only refs are checked the network cares about the GraphHelper#refs
method has no need to reject those, simplifying the method.

Closes gitlab-org/gitaly#880

Closes gitaly#915

Uses `list_commits_by_oid` on the CommitService, to request the needed
commits for pipelines. These commits are needed to display the user that
created the commit and the commit title.

This includes fixes for tests failing that depended on the commit
being `nil`. However, now these are batch loaded, this doesn't happen
anymore and the commits are an instance of BatchLoader.

- Added tests for Git::Commit.shas_with_signatures

Closes gitaly#737

Now, when requesting a commit from the Repository model, the results are
not cached. This means we're fetching the same commit by oid multiple times
during the same request. To prevent us from doing this, we now cache
results. Caching is done only based on object id (aka SHA).

Given we cache on the Repository model, results are scoped to the
associated project, eventhough the change of two repositories having the
same oids for different commits is small.

This helps keep the abstraction layers simpler, and also keep the
interface of those methods consistent, in case of implementation
changes.

the rugged versions up to 0.26.0b3 had a bug concerning the signature
extraction. The extracted signature was not always the same, probably
due to a buffer (overflow) issue in libgit.

see https://github.com/libgit2/rugged/issues/608

Closes gitaly#396