Tab complete commit messages

[georgebrock]

Git allows a revision to be specified by giving a substring of the commit message's first line prefixed with :/, e.g. :/refactor or :/"refactor tab" would match messages like refactor tab completion.

We could provide tab completion for this!

[georgebrock]

Bumping this from v0.14 to v1.0: this is still a cool feature, but I don't want to delay releasing some of the big tab completion improvements that we've done recently for these kinds of bells and whistles.