Add --no-commit option to add, update, and remove

[mattmccutchen]

I'd like to be able to add, update, or remove mirrors in the same commit as other related changes. I propose that the add, update, and remove commands could take a --no-commit option to stage the changes without automatically committing. This option should also allow the working tree to have local changes as long as they won't interfere with the command.

I don't think this is a big enough deal for me to put in the work to implement it right now (I can just make multiple commits and then squash them), but I wanted to go ahead and log the enhancement proposal.

[realityforge]

This would be a useful change, often we rebase changes to effect a similar change