Handle just pressing enter

If you just press enter, this will run a `git status`.

Unsure about this for the long-term. Seems lazy and unlike me to do this
kind of convenience thing. sh doesn't do it, afterall.
Support repos with only modifications

Problem I'm running into: git repos can be in many states, and I'm
struggling to catch them all. Just disovered the state where a file has
been modified but no files are added or deleted, and modified the code
to account for that.