status: print branch info with --porcelain=v2 --branch

Expand porcelain v2 output to include branch and tracking
branch information. This includes the commit id, the branch,
the upstream branch, and the ahead and behind counts.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wt-status.h

@@ -80,6 +80,7 @@ struct wt_status {
 	int hints;
 
 	enum wt_status_format status_format;
+	unsigned char sha1_commit[GIT_SHA1_RAWSZ]; /* when not Initial */
 
 	/* These are computed during processing of the individual sections */
 	int commitable;