Git should use --git-dir and --work-tree not changing directory

[ndmitchell]

If the git process used gitdir/worktree arguments instead of changing process directory it would be easier to debug unexpected failures because you could copy/paste the command lines anywhere. See http://stackoverflow.com/questions/1386291/git-git-dir-not-working-as-expected for advice on how to use it.

[ndmitchell]

#1578 alleviates this a bit, but using just flags would still be more convenient.

[mgsloan]

Makes sense to me, I agree it's nice when the command lines in the verbose log can be copy-pasted.

[snoyberg]

With the new Pantry approach, I don't think this is relevant on master anymore.