rebase: add the --gpg-sign option

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-rebase--merge.sh b/git-rebase--merge.sh
index e7d96de..5381857 100644
--- a/git-rebase--merge.sh
+++ b/git-rebase--merge.sh
@@ -27,7 +27,7 @@
 	cmt=`cat "$state_dir/current"`
 	if ! git diff-index --quiet --ignore-submodules HEAD --
 	then
-		if ! git commit --no-verify -C "$cmt"
+		if ! git commit ${gpg_sign_opt:+"$gpg_sign_opt"} --no-verify -C "$cmt"
 		then
 			echo "Commit failed, please do not call \"git commit\""
 			echo "directly, but instead do one of the following: "