not support bare repository

[Aimeast]

Hi, sometimes in server side we need manage bare repository, but when I <cd> to xx.git folder, no state display, posh-git not working in bare repository.

[dahlbyk]

I know this worked once upon a time, but I'm pretty sure it has been broken since 0dc174d.

[rkeithhill]

@dahlbyk I'm looking at that commit. Is there a reason why this was changed to not use git rev-parse --git-dir? I'm tempted to just change the code back perhaps with a resolve-path thrown in i.e. if you are in the root of a bare repo that git command returns ..