Git "raw commit" feature should use git mbox format (BB-7813)

On the single commit screen, there is a "raw commit" link. Currently when using that link in a Mercurial repository, it uses the "hg export" format, which includes commit message, authorship, date, time and other metadata.

The git raw commit feature, however, shows a plain old diff: https://bitbucket.org/intgr/pg_journal/commits/fc6d9ee81c812b59dbbbe57c7f22fe3d1b1f1a31/raw/

I believe it would be better to use the git mbox export format, produced by "git format-patch" – which also preserves message, author, date etc.