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.

%% Should show unknown
? unknown
removing b
%% Should show unknown and b removed
R b
? unknown
%% Should show a and unknown
a
unknown