color: add --color switch
--color works like GNU grep --color. --color=auto only colorizes output
for non-dumb terminals that are TTYs.
hg init test
cd test
echo foo > a
hg ci -Ama
hg up -r0000
echo bar > a
echo % should issue warning
hg ci -Amb
hg up -r0000
echo stuffy > a
echo % should not issue warning
hg ci -q -Amc
hg up -r0000
echo crap > a
hg branch testing
echo % should not issue warning
hg ci -q -Amd