default color.status.branch to "same as header"

This gives it the same behavior as we had prior to 1d28232
(status: show branchname with a configurable color).

To do this we need the concept of a "NIL" color, which is
provided by color.[ch]. The implementation is very simple;
in particular, there are no precautions taken against code
accidentally printing the NIL. This should be fine in
practice because:

  1. You can't input a NIL color in the config, so it must
     come from the in-code defaults. Which means it is up
     the client code to handle the NILs it defines.

  2. If we do ever print a NIL, it will be obvious what the
     problem is, and the bug can be fixed.

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

color.h

@@ -43,6 +43,9 @@
 #define GIT_COLOR_BG_MAGENTA	"\033[45m"
 #define GIT_COLOR_BG_CYAN	"\033[46m"
 
+/* A special value meaning "no color selected" */
+#define GIT_COLOR_NIL "NIL"
+
 /*
  * This variable stores the value of color.ui
  */
@@ -62,4 +65,6 @@ int color_fprintf(FILE *fp, const char *color, const char *fmt, ...);
 __attribute__((format (printf, 3, 4)))
 int color_fprintf_ln(FILE *fp, const char *color, const char *fmt, ...);
 
+int color_is_nil(const char *color);
+
 #endif /* COLOR_H */