add status.relativePaths config variable

The output of git-status was recently changed to output relative
paths. Setting this variable to false restores the old behavior for
any old-timers that prefer it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin-commit.c b/builtin-commit.c
index 2ec8223..19297ac 100644
--- a/builtin-commit.c
+++ b/builtin-commit.c
@@ -285,7 +285,8 @@
 	struct wt_status s;
 
 	wt_status_prepare(&s);
-	s.prefix = prefix;
+	if (wt_status_relative_paths)
+		s.prefix = prefix;
 
 	if (amend) {
 		s.amend = 1;