Use "working tree" instead of "working directory" for git status

Working directory can be easily confused with the current directory.
In one of my patches I already updated the usage of working directory
with working tree for the man page but I noticed that git status also
uses this incorrect term.
Signed-off-by: NLars Vogel <Lars.Vogel@vogella.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

wt-status.c

@@ -1554,7 +1554,7 @@ void wt_status_print(struct wt_status *s)
 			else
 				printf(_("nothing to commit\n"));
 		} else
-			printf(_("nothing to commit, working directory clean\n"));
+			printf(_("nothing to commit, working tree clean\n"));
 	}
 }