Do not fork PAGER=cat

Unless the user has a nonstandard "cat" command that does not
meow like a cat, this should not break anything and would save an
extra pipe.

Signed-off-by: Junio C Hamano <junkio@cox.net>

pager.c

@@ -20,7 +20,7 @@ void setup_pager(void)
 		return;
 	if (!pager)
 		pager = "less";
-	else if (!*pager)
+	else if (!*pager || !strcmp(pager, "cat"))
 		return;
 
 	if (pipe(fd) < 0)