wt-status: refactor detached HEAD analysis

wt_status_print() is the only caller of wt_status_get_detached_from().
The latter performs most of the analysis of a detached HEAD, including
finding state->detached_from; the caller checks whether the detached
HEAD is still at state->detached_from or has moved away.

Move that last bit of analysis to wt_status_get_detached_from(), too,
and store the boolean result in state->detached_at.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wt-status.h

@@ -84,6 +84,7 @@ struct wt_status_state {
 	int cherry_pick_in_progress;
 	int bisect_in_progress;
 	int revert_in_progress;
+	int detached_at;
 	char *branch;
 	char *onto;
 	char *detached_from;