format-patch: page output with --stdout

Pass output through the pager if format-patch is run with --stdout. This
saves the user the trouble of running git with '-p' or piping through a
pager.

setup_pager() already checks if stdout is a tty, so we don't have to
worry about behaviour if the user redirects/pipes stdout. Paging can
also be disabled with the config

  [pager]
      format-patch = false

Add tests to check for these behaviour.
Helped-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NTay Ray Chuan <rctay89@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1159,6 +1159,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 
 	if (!use_stdout)
 		output_directory = set_outdir(prefix, output_directory);
+	else
+		setup_pager();
 
 	if (output_directory) {
 		if (use_stdout)

t/t4014-format-patch.sh

@@ -6,6 +6,7 @@
 test_description='various format-patch tests'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-terminal.sh
 
 test_expect_success setup '
 
@@ -686,4 +687,26 @@ test_expect_success 'format-patch --signature="" supresses signatures' '
 	! grep "^-- \$" output
 '
 
+test_expect_success TTY 'format-patch --stdout paginates' '
+	rm -f pager_used &&
+	(
+		GIT_PAGER="wc >pager_used" &&
+		export GIT_PAGER &&
+		test_terminal git format-patch --stdout --all
+	) &&
+	test_path_is_file pager_used
+'
+
+ test_expect_success TTY 'format-patch --stdout pagination can be disabled' '
+	rm -f pager_used &&
+	(
+		GIT_PAGER="wc >pager_used" &&
+		export GIT_PAGER &&
+		test_terminal git --no-pager format-patch --stdout --all &&
+		test_terminal git -c "pager.format-patch=false" format-patch --stdout --all
+	) &&
+	test_path_is_missing pager_used &&
+	test_path_is_missing .git/pager_used
+'
+
 test_done