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.

adding bar
adding foo
% state dump
a   0         -1 baz
n 644          0 foo
r   0          0 bar
% status
A baz
R bar
C foo
% state dump
n 666         -1 bar
n 666         -1 foo
% status
! bar
? baz
C foo