Dumping the user into a dirtied working tree after a failed rebase
attempt can be confusing, no matter how much contextual explanation
we provide when doing so. By default, run `git rebase --abort`
automatically so as to clean up from a failed test rebase, and then
let the user rebase again on their own if that's the state they want
to be in. Add a -K/--keep-rebase option to get the old behavior, and
mention it when we automatically abort in case the user wants to
just have git-review redo the rebase for them and leave things in
that incomplete state.
Change-Id: I7d7bfca1623a71a9b4fe445360d94fd6b039f040
da1e8a05f4