Support serving bare repositories

Created by: andir

This is in response to my own issue (#634) where I was asking for support for serving bare repositories from a directory.

This PR has a couple of changes. Some of them can be seen as a refactoring of sorts. The comments in the original implementation were a bit misleading as they appeared to be the other way around (compared to what the tests wanted). I took the liberty to rewrite them a bit. All of the original tests still pass. I've added one case where a bare repository is created (by suffixing the name of a repo with .bare) which seems to work. I am not happy about the repository name suffix hack that I had to add but it also doesn't seem like it is worth the effort to refactor the entire test machinery just because of this.

I am open for feedback on this.

cc @mrnugget & @keegancsmith who where very responsive to my issue