built-in rebase --skip/--abort: clean up stale .git/<name> files
The scripted version of the rebase used to execute `git reset --hard`
when skipping or aborting. When we ported this to C, we did update the
worktree and some reflogs, but we failed to imitate `git reset --hard`'s
behavior regarding files in .git/ such as MERGE_HEAD.
Let's address this oversight.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>