status/commit: make sure --porcelain is not affected by user-facing config

The recent addition of status.branch started affecting what is shown
when "git status --porcelain" is run by mistake.  Identify the
configuration items that should be ignored under "--porcelain"
option, introduce a "deferred config" mechanism to keep the values
read from the configuration, and decide what value to use only after
we read both from configuration and command line.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

wt-status.c

@@ -127,6 +127,7 @@ void wt_status_prepare(struct wt_status *s)
 	s->change.strdup_strings = 1;
 	s->untracked.strdup_strings = 1;
 	s->ignored.strdup_strings = 1;
+	s->show_branch = -1;  /* unspecified */
 }
 
 static void wt_status_print_unmerged_header(struct wt_status *s)