color: handle 'ui.plain()' directly in mode setup
If 'ui.plain()' is set we should not colorize. We move that logic into the
function that determine and setup the color mode. As all other code respect
the resulting mode this will be equivalent.
#require serve killdaemons
$ 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