Allow braid add/update if there is no uncommitted changes in the target path.

[realityforge]

Sometimes a user wants to braid update an existing upstream or braid add a new upstream and if there is no local changes in the local path this should be possible but it is not currently. It may require a little bit of tricky coding and there is the risk that in the case that braid update causes a conflict you may lose your local index but this may be acceptable.

[mattmccutchen]

This would commit the change to the mirror (and .braids) and leave any other uncommitted changes alone, right? I didn't think of this design; then this feature would become independent of the --no-commit option contemplated in #36.

There should also be no pre-existing uncommitted change to .braids, right?

[realityforge]

Yep on all accounts ;)