git-merge-ours: make sure our index matches HEAD

git-merge expects this check to be done appropriately by the
merge strategy backends.  In the case of merge-ours strategy,
the resulting tree comes what we have in the index file, so it
must match the current HEAD; otherwise it would not be "ours"
merge.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge-ours.sh

@@ -4,4 +4,11 @@
 #
 # Pretend we resolved the heads, but declare our tree trumps everybody else.
 #
+
+# We need to exit with 2 if the index does not match our HEAD tree,
+# because the current index is what we will be committing as the
+# merge result.
+
+test "$(git-diff-index --cached --name-status HEAD)" = "" || exit 2
+
 exit 0