status: respect "-b" for porcelain format

There is no reason not to, as the user has to explicitly ask
for it, so we are not breaking compatibility by doing so. We
can do this simply by moving the "show_branch" flag into
the wt_status struct. As a bonus, this saves us from passing
it explicitly, simplifying the code.

Signed-off-by: Jeff King <peff@peff.net>
diff --git a/wt-status.c b/wt-status.c
index b5305ae..bc268ce 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -918,11 +918,11 @@
 	fputc(s->null_termination ? '\0' : '\n', s->fp);
 }
 
-void wt_shortstatus_print(struct wt_status *s, int show_branch)
+void wt_shortstatus_print(struct wt_status *s)
 {
 	int i;
 
-	if (show_branch)
+	if (s->show_branch)
 		wt_shortstatus_print_tracking(s);
 
 	for (i = 0; i < s->change.nr; i++) {
@@ -955,5 +955,5 @@
 	s->use_color = 0;
 	s->relative_paths = 0;
 	s->prefix = NULL;
-	wt_shortstatus_print(s, 0);
+	wt_shortstatus_print(s);
 }