chgserver: update the umask cache before each run
posix.py uses a global variable to store the umask value resulting in caching of
it when using chg. We need to update it before each command run as the umask can
change between commands.
This fixes test-inherit-mode.t with chg.
#require serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ "$PYTHON" "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py