Allow editing of commit messages

As part of &4983 (closed) and gitlab-com/gl-infra&351 (closed) we need the ability to edit commit messages. Since GitLab needs to use Gitaly for this, we'll need to extend Gitaly to support this to some degree.

Specifically, we'll need an RPC/API call that takes as input a project, commit SHA, and a new commit message. The call will then update the message of that commit to the new message. The response would then be the new commit object, allowing GitLab Rails to directly use that data for displaying the new commit details.

&4941 talks about full rebasing support, but that may be a step too far. With that said, editing a commit message is a subset of rebasing, so I suspect both the Delivery and Gitaly epics would use the same foundation.

If a Git commit is signed using GPG, editing shouldn't be allowed. A signed commit can't be re-signed using GitLab, so editing them would result in users no longer being able to verify the author of the commit truly is the right author.