"git -p cmd" to page anywhere

This allows you to say:

	git -p diff v2.6.16-rc5..

and the command pipes the output of any git command to your pager.

[jc: this resurrects a month old RFC patch with improvement
 suggested by Linus to call it --paginate instead of --less.]

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

cache.h

@@ -382,6 +382,7 @@ extern int receive_keep_pack(int fd[2], const char *me, int quiet, int);
 
 /* pager.c */
 extern void setup_pager(void);
+extern int pager_in_use;
 
 /* base85 */
 int decode_85(char *dst, char *line, int linelen);