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.
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..