parse_color: recognize "no$foo" to clear the $foo attribute

parse_color: recognize "no$foo" to clear the $foo attribute

You can turn on ANSI text attributes like "reverse" by
putting "reverse" in your color spec. However, you cannot
ask to turn reverse off.

For common cases, this does not matter. You would turn on
"reverse" at the start of a colored section, and then clear
all attributes with a "reset". However, you may wish to turn
on some attributes, then selectively disable others. For
example:

  git log --format="%C(bold ul yellow)%h%C(noul) %s"

underlines just the hash, but without the need to re-specify
the rest of the attributes. This can also help third-party
programs, like contrib/diff-highlight, that want to turn
some attribute on/off without disrupting existing coloring.

Note that some attribute specifications are probably
nonsensical (e.g., "bold nobold"). We do not bother to flag
such constructs, and instead let the terminal sort it out.

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