color.ui docs & add color.ui=isatty

I started writing this for the reasons explained in 4/4, but the justification there is a bit of a stretch. Where we'd realistically like to diverge color.ui=auto and color.pager=false, but since I wrote it already maybe some people will come out of the woodworks telling me that this is what they've always wanted for whatever reason.

Maybe if we don't like 4/4 we should take 3/4 because we might want another such option in the future, but that's probably overly hedging our bets, still. I really don't like this pattern that we have a multi-value config option that dies instead of warns on unknown future values, so I'm leaning towards saying that should be accepted to git.

But while I was working towards 4/4 I did some nice fixes in [12]/4. I think those should go in regardless, so they're non-RFC.

Ævar Arnfjörð Bjarmason (4):
config doc: move color.ui documentation to one place config doc: clarify "to a terminal" in color.ui color.ui config: don't die on unknown values color.ui config: add "isatty" setting

 Documentation/config.txt | 100 +++++++++++++++++++++------------------
 color.c                  |  25 ++++++++--
 color.h                  |   1 +
 t/t7006-pager.sh         |  31 ++++++++++++
 4 files changed, 107 insertions(+), 50 deletions(-)

2.17.0.290.gded63e768a