force_object_loose: Fix memory leak

read_packed_sha1 expectes its caller to free the buffer it returns, which
force_object_loose didn't do.

This leak is eventually triggered by "git gc", when it is manually invoked
or there are too many packs around, making gc totally unusable when there
are lots of unreachable objects.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2322,14 +2322,18 @@ int force_object_loose(const unsigned char *sha1, time_t mtime)
 	enum object_type type;
 	char hdr[32];
 	int hdrlen;
+	int ret;
 
 	if (has_loose_object(sha1))
 		return 0;
 	buf = read_packed_sha1(sha1, &type, &len);
 	if (!buf)
 		return error("cannot read sha1_file for %s", sha1_to_hex(sha1));
 	hdrlen = sprintf(hdr, "%s %lu", typename(type), len) + 1;
-	return write_loose_object(sha1, hdr, hdrlen, buf, len, mtime);
+	ret = write_loose_object(sha1, hdr, hdrlen, buf, len, mtime);
+	free(buf);
+
+	return ret;
 }
 
 int has_pack_index(const unsigned char *sha1)