git-merge: warn when -m provided on a fast forward

Warn the user that the "-m" option is ignored in the case of a fast
forward.  That may save some confusion in the case where the user
doesn't know about fast forwards yet and may not realize that the
behavior here is intentional.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge.sh

@@ -294,7 +294,12 @@ f,*)
 	git-update-index --refresh 2>/dev/null
 	new_head=$(git-rev-parse --verify "$1^0") &&
 	git-read-tree -v -m -u --exclude-per-directory=.gitignore $head "$new_head" &&
-	finish "$new_head" "Fast forward" || exit
+	msg="Fast forward"
+	if test -n "$have_message"
+	then
+		msg="$msg (no commit created; -m option ignored)"
+	fi
+	finish "$new_head" "$msg" || exit
 	dropsave
 	exit 0
 	;;