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.
$ 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