Allow pager of diff command be enabled/disabled

See for example, status and show commands. Besides,
Documentation/RelNotes-1.6.0.txt mentions that pager.<cmd>
can be used to enable/disable paging behavior per command.
Signed-off-by: NAlex Riesen <raa.lkml@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-diff.c

@@ -296,7 +296,8 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 	 * If the user asked for our exit code then don't start a
 	 * pager or we would end up reporting its exit code instead.
 	 */
-	if (!DIFF_OPT_TST(&rev.diffopt, EXIT_WITH_STATUS))
+	if (!DIFF_OPT_TST(&rev.diffopt, EXIT_WITH_STATUS) &&
+	    check_pager_config("diff") != 0)
 		setup_pager();
 
 	/*

builtin.h

@@ -18,6 +18,7 @@ extern int fmt_merge_msg(int merge_summary, struct strbuf *in,
 	struct strbuf *out);
 extern int commit_tree(const char *msg, unsigned char *tree,
 		struct commit_list *parents, unsigned char *ret);
+extern int check_pager_config(const char *cmd);
 
 extern int cmd_add(int argc, const char **argv, const char *prefix);
 extern int cmd_annotate(int argc, const char **argv, const char *prefix);