completion: suggest sequencer commands for revert

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>
Acked-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2292,6 +2292,11 @@ _git_reset ()
 
 _git_revert ()
 {
+	local dir="$(__gitdir)"
+	if [ -f "$dir"/REVERT_HEAD ]; then
+		__gitcomp "--continue --quit --abort"
+		return
+	fi
 	case "$cur" in
 	--*)
 		__gitcomp "--edit --mainline --no-edit --no-commit --signoff"