t3701: add a test for interactive.diffFilter

This feature was added in 01143847db (add--interactive:
allow custom diff highlighting programs, 2016-02-27) but
never tested. Let's add a basic test.

Note that we only apply the filter when color is enabled,
so we have to use test_terminal. This is an open limitation
explicitly mentioned in the original commit. So take this
commit as testing the status quo, and not making a statement
on whether we'd want to enhance that in the future.

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

t/t3701-add-interactive.sh

@@ -392,6 +392,18 @@ test_expect_success TTY 'diffs can be colorized' '
 	grep "$(printf "\\033")" output
 '
 
+test_expect_success TTY 'diffFilter filters diff' '
+	git reset --hard &&
+
+	echo content >test &&
+	test_config interactive.diffFilter "sed s/^/foo:/" &&
+	printf y | test_terminal git add -p >output 2>&1 &&
+
+	# avoid depending on the exact coloring or content of the prompts,
+	# and just make sure we saw our diff prefixed
+	grep foo:.*content output
+'
+
 test_expect_success 'patch-mode via -i prompts for files' '
 	git reset --hard &&