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.

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "1000000 0"
hg verify
hg parents
hg status
hg rollback
hg verify
hg parents
hg status