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
# test command parsing and dispatch

hg init a
cd a
echo a > a
hg ci -Ama -d '0 0'

echo "# missing arg"
hg cat

echo '% [defaults]'
hg cat a
cat > $HGRCPATH <<EOF
[defaults]
cat = -v
EOF
hg cat a

echo '% no repo'
cd ..
hg cat

exit 0