color: use GIT_COLOR_* instead of numeric constants

Long ago Git's decision to show color for a subsytem was stored in a
tri-state variable: it could be true (1), false (0), or unknown (-1).
But since daa0c3d971 (color: delay auto-color decision until point of
use, 2011-08-17) we want to carry around a new state, "auto", which
bases the decision on the tty-ness of stdout (rather than collapsing
that "auto" state to a true/false immediately).

That commit introduced a set of GIT_COLOR_* defines to represent each
state: UNKNOWN, ALWAYS, NEVER, and AUTO. But it only used the AUTO
value, and left alone code using bare 0/1/-1 values. And of course since
then we've grown many new spots that use those bare values.

Let's switch all of these to use the named constants. That should make
the code a bit easier to read, as it is more obvious that we're
representing a color decision.

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

advice.c

@@ -7,7 +7,7 @@
 #include "help.h"
 #include "string-list.h"
 
-static int advice_use_color = -1;
+static int advice_use_color = GIT_COLOR_UNKNOWN;
 static char advice_colors[][COLOR_MAXLEN] = {
 	GIT_COLOR_RESET,
 	GIT_COLOR_YELLOW,	/* HINT */