tests: don't run test-update-atomic.t on chg
I am unable to find a good way to make `showwrites` extension in it to work with
chg. Also putting the use of showwrites inside `if no-chg` will defeat the
purpose of test, so I just made the test no-op on chg.
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF