checkout: fix memory leak

When "git checkout -m" does an in-core three-way merge to carry
local modifications forward to check out a different branch, the
code forgot to free the updated contents it has in-core.
Noticed-by: NRené Scharfe <l.s.r@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -247,6 +247,7 @@ static int checkout_merged(int pos, const struct checkout *state)
 	if (write_sha1_file(result_buf.ptr, result_buf.size,
 			    blob_type, oid.hash))
 		die(_("Unable to add merge result for '%s'"), path);
+	free(result_buf.ptr);
 	ce = make_cache_entry(mode, oid.hash, path, 2, 0);
 	if (!ce)
 		die(_("make_cache_entry failed for path '%s'"), path);