pager: make pager_program a file-local static

This variable is only ever used by the routines in pager.c,
and other parts of the code should always use those routines
(like git_pager()) to make decisions about which pager to
use. Let's reduce its scope to prevent accidents.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pager.c

@@ -7,6 +7,7 @@
 #endif
 
 static struct child_process pager_process = CHILD_PROCESS_INIT;
+static const char *pager_program;
 
 static void wait_for_pager(int in_signal)
 {