Add GitRepo#owner, for easier stubbing.
Set the GitFS owner's git name and email if they're not set already.

Git requires these to be set in order to commit, and they're often not set on a fresh system.
Use #shell! in destructive GitRepo methods, so failing commands raise.

This is a breaking change, but it should break for the better. That is, if a #checkout!, #branch!, etc is failing in your dep, this patch will make it raise, but since it was already failing, failing fast is preferable.
Merge branch 'git-repo-exceptions' into git-fs-initing
Don't fail when GitFS has nothing to commit; that's not an error.
Log when snapshotting and setting git name/email.
Merge branch 'git-fs-initing'