want_color: automatically fallback to color.ui

All of the "do we want color" flags default to -1 to
indicate that we don't have any color configured. This value
is handled in one of two ways:

  1. In porcelain, we check early on whether the value is
     still -1 after reading the config, and set it to the
     value of color.ui (which defaults to 0).

  2. In plumbing, it stays untouched as -1, and want_color
     defaults it to off.

This works fine, but means that every porcelain has to check
and reassign its color flag. Now that want_color gives us a
place to put this check in a single spot, we can do that,
simplifying the calling code.

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

color.h

@@ -58,11 +58,6 @@ struct strbuf;
 #define GIT_COLOR_ALWAYS 1
 #define GIT_COLOR_AUTO   2
 
-/*
- * This variable stores the value of color.ui
- */
-extern int git_use_color_default;
-
 /* A default list of colors to use for commit graphs and show-branch output */
 extern const char *column_colors_ansi[];
 extern const int column_colors_ansi_max;