pretty: respect color settings for %C placeholders

pretty: respect color settings for %C placeholders

The color placeholders have traditionally been
unconditional, showing colors even when git is not otherwise
configured to do so. This was not so bad for their original
use, which was on the command-line (and the user could
decide at that moment whether to add colors or not). But
these days we have configured formats via pretty.*, and
those should operate correctly in multiple contexts.

In 3082517 (log --format: teach %C(auto,black) to respect
color config, 2012-12-17), we gave an extended placeholder
that could be used to accomplish this. But it's rather
clunky to use, because you have to specify it individually
for each color (and their matching resets) in the format.
We shied away from just switching the default to auto,
because it is technically breaking backwards compatibility.

However, there's not really a use case for unconditional
colors. The most plausible reason you would want them is to
redirect "git log" output to a file. But there, the right
answer is --color=always, as it does the right thing both
with custom user-format colors and git-generated colors.

So let's switch to the more useful default. In the
off-chance that somebody really does find a use for
unconditional colors without wanting to enable the rest of
git's colors, we provide a new %C(always,...) to enable the
old behavior. And we can remind them of --color=always in
the documentation.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>