bash: Support git-rebase -m continuation completion.

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>

contrib/completion/git-completion.bash

@@ -675,7 +675,7 @@ _git_push ()
 _git_rebase ()
 {
 	local cur="${COMP_WORDS[COMP_CWORD]}"
-	if [ -d .dotest ]; then
+	if [ -d .dotest ] || [ -d .git/.dotest-merge ]; then
 		__gitcomp "--continue --skip --abort"
 		return
 	fi