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 print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'