setup: filter out devel-warn messages from system hg
If we're going to use the user's installed and configured hg command
(which we do since
8b20338b989e), we should prevent devel-warn messages
from interfering with locating it.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown