Apparently `git-rebase -m` uses a metadata directory within .git
(.git/.dotest-merge) rather than .dotest used by git-am (and
git-rebase without the -m option). This caused the completion code
to not offer --continue, --skip or --abort when working within a
`git-rebase -m` session.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>