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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

color.c

@@ -211,3 +211,8 @@ int color_fprintf_ln(FILE *fp, const char *color, const char *fmt, ...)
 	va_end(args);
 	return r;
 }
+
+int color_is_nil(const char *c)
+{
+	return !strcmp(c, "NIL");
+}

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 */

wt-status.c

@@ -21,12 +21,15 @@ static char default_wt_status_colors[][COLOR_MAXLEN] = {
 	GIT_COLOR_RED,    /* WT_STATUS_UNMERGED */
 	GIT_COLOR_GREEN,  /* WT_STATUS_LOCAL_BRANCH */
 	GIT_COLOR_RED,    /* WT_STATUS_REMOTE_BRANCH */
-	GIT_COLOR_NORMAL, /* WT_STATUS_ONBRANCH */
+	GIT_COLOR_NIL,    /* WT_STATUS_ONBRANCH */
 };
 
 static const char *color(int slot, struct wt_status *s)
 {
-	return s->use_color > 0 ? s->color_palette[slot] : "";
+	const char *c = s->use_color > 0 ? s->color_palette[slot] : "";
+	if (slot == WT_STATUS_ONBRANCH && color_is_nil(c))
+		c = s->color_palette[WT_STATUS_HEADER];
+	return c;
 }
 
 void wt_status_prepare(struct wt_status *s)