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>

builtin-merge-ours.c

@@ -0,0 +1,28 @@
+/*
+ * Implementation of git-merge-ours.sh as builtin
+ *
+ * Copyright (c) 2007 Thomas Harning Jr
+ * Original:
+ * Original Copyright (c) 2005 Junio C Hamano
+ *
+ * Pretend we resolved the heads, but declare our tree trumps everybody else.
+ */
+#include "git-compat-util.h"
+#include "builtin.h"
+
+static const char *diff_index_args[] = {
+	"diff-index", "--quiet", "--cached", "HEAD", "--", NULL
+};
+#define NARGS (ARRAY_SIZE(diff_index_args) - 1)
+
+int cmd_merge_ours(int argc, const char **argv, const char *prefix)
+{
+	/*
+	 * 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.
+	 */
+	if (cmd_diff_index(NARGS, diff_index_args, prefix))
+		exit(2);
+	exit(0);
+}