Show Git Information when Compiling a Path Dependency

[nixpulvis]

I have a few projects where I maintain forks of dependencies locally. I often work on them independently of each other and the projects they are used in. When compiling a project it'd be nice to know what branch of local paths I just compiled. Other git info might be useful, but branches are the key for me.

[alexcrichton]

This is somewhat related to #1743, but we should in theory basically have this information already, so should just be a matter of figuring out what to display

[nixpulvis]

I'd be happy enough with the logic of, if the path is a git repository then show the branch otherwise do what we do now.