[git-rebase-chain] Fix some issues and missing output when rebasing.

We now strip the "refs/heads/", in order to fix the actual updating of
branches when rebasing. Turns out using "refs/heads/" as a prefix will
do the rebase, but not update the branch location.

We now also output the results from git rebase --continue, --skip,
and --abort.
[git-rebase-chain] Add --debug for showing debug output.

Debug output is no longer shown by default. Instead, --debug must be
passed to see this output.
[git-rebase-chain] Always delete state when using --abort.

If git rebase --abort failed, we wouldn't delete the rebase-chain state.
We now delete this first in order to prevent issues with future rebases.