git-merge-ours: make it a builtin.

Except that this fixes a longstanding corner case bug by
tightening the way underlying diff-index command is run, it is
functionally equivalent to the scripted version.

Signed-off-by: Thomas Harning Jr <harningt@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-merge-ours.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+#
+# Copyright (c) 2005 Junio C Hamano
+#
+# 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.
+
+git diff-index --quiet --cached HEAD || exit 2
+
+exit 0