diff-highlight: check diff-highlight exit status in tests

When testing diff-highlight, we pipe the output through a sanitizing
function. This loses the exit status of diff-highlight itself, which
could mean we are missing cases where it crashes or exits unexpectedly.
Use an extra tempfile to avoid the pipe.

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

Author: peff

contrib/diff-highlight/t/t9400-diff-highlight.sh

@@ -41,8 +41,10 @@ dh_test () {
 		git show >commit.raw
 	} >/dev/null &&
 
-	"$DIFF_HIGHLIGHT" <diff.raw | test_strip_patch_header >diff.act &&
-	"$DIFF_HIGHLIGHT" <commit.raw | test_strip_patch_header >commit.act &&
+	"$DIFF_HIGHLIGHT" <diff.raw >diff.hi &&
+	test_strip_patch_header <diff.hi >diff.act &&
+	"$DIFF_HIGHLIGHT" <commit.raw >commit.hi &&
+	test_strip_patch_header <commit.hi >commit.act &&
 	test_cmp patch.exp diff.act &&
 	test_cmp patch.exp commit.act
 }