[git-rebase-chain] Fix loading remaining branches when empty.

If the list of remaining branches was empty, it'd get turned into a list
of one empty string, which would break the rebase operations. It now
properly handles the case of an empty list of rebases, terminating the
rebase-chain operation cleanly.
[git-rebase-chain] Don't run git rebase --continue if there's no rebase state.

Passing --continue to `git rebase-chain` would unconditionally attempt
to run `git rebase --continue`. However, there may not be any rebase
state left, so check for that before attempting to run
`git rebase --continue`.

This works around an issue I had when debugging the program. It probably
shouldn't matter in real life, but it's at least a bit more complete.