Only set mode of new patch if the target file was removed before.
If the file is writable by the user, but owned by a different user, the
chmod will otherwise fail with "Operation not permitted".
Additionally make very sure that the file is only written if either the number
of links is <= 1 or the file was successfully removed.
Maybe this minimal COW code should be replaced by something from util.
foo renamed from bar:26d3ca0dfd18e44d796b564e38dd173c9668d3a9
rev offset length base linkrev nodeid p1 p2
0 0 6 0 0 26d3ca0dfd18 000000000000 000000000000
1 6 7 1 1 d267bddd54f7 26d3ca0dfd18 000000000000