status: show 'revert' state and status hint

This is the logical equivalent for "git status" of 3ee4452 (bash: teach
__git_ps1 about REVERT_HEAD).

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wt-status.h

@@ -80,6 +80,7 @@ struct wt_status_state {
 	int rebase_interactive_in_progress;
 	int cherry_pick_in_progress;
 	int bisect_in_progress;
+	int revert_in_progress;
 	char *branch;
 	char *onto;
 	char *detached_from;