t7511: avoid use of reserved filename on Windows.

PRN is a special filename on Windows to send data to the printer. As
this is generated during test 3 substitute an alternate prefix to avoid this.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: patthoyts

t/t7511-status-index.sh

@@ -24,7 +24,7 @@ check() {
 
 check  1
 check  2 p
-check  3 pr
+check  3 px
 check  4 pre
 check  5 pref
 check  6 prefi