status: show branchname with a configurable color

You can tell "git status" to paint the name of the current branch in its
output (the line that says "On branch ...") by setting the configuration
variable color.status.branch; it is by default turned off.

Signed-off-by: Aleksi Aalto <aga@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -984,6 +984,8 @@ static int parse_status_slot(const char *var, int offset)
 {
 	if (!strcasecmp(var+offset, "header"))
 		return WT_STATUS_HEADER;
+	if (!strcasecmp(var+offset, "branch"))
+		return WT_STATUS_ONBRANCH;
 	if (!strcasecmp(var+offset, "updated")
 		|| !strcasecmp(var+offset, "added"))
 		return WT_STATUS_UPDATED;