ignore unknown color configuration

ignore unknown color configuration

When parsing the config file, if there is a value that is
syntactically correct but unused, we generally ignore it.
This lets non-core porcelains store arbitrary information in
the config file, and it means that configuration files can
be shared between new and old versions of git (the old
versions might simply ignore certain configuration).

The one exception to this is color configuration; if we
encounter a color.{diff,branch,status}.$slot variable, we
die if it is not one of the recognized slots (presumably as
a safety valve for user misconfiguration). This behavior
has existed since 801235c (diff --color: use
$GIT_DIR/config, 2006-06-24), but hasn't yet caused a
problem. No porcelain has wanted to store extra colors, and
we once a color area (like color.diff) has been introduced,
we've never changed the set of color slots.

However, that changed recently with the addition of
color.diff.func. Now a user with color.diff.func in their
config can no longer freely switch between v1.6.6 and older
versions; the old versions will complain about the existence
of the variable.

This patch loosens the check to match the rest of
git-config; unknown color slots are simply ignored. This
doesn't fix this particular problem, as the older version
(without this patch) is the problem, but it at least
prevents it from happening again in the future.

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