add test_cmp function for test scripts

Many scripts compare actual and expected output using
"diff -u". This is nicer than "cmp" because the output shows
how the two differ. However, not all versions of diff
understand -u, leading to unnecessary test failure.

This adds a test_cmp function to the test scripts and
switches all "diff -u" invocations to use it. The function
uses the contents of "$GIT_TEST_CMP" to compare its
arguments; the default is "diff -u".

On systems with a less-capable diff, you can do:

  GIT_TEST_CMP=cmp make test

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

t/t4024-diff-optimize-common.sh

@@ -150,7 +150,7 @@ test_expect_success 'diff -U0' '
 	do
 		git diff -U0 file-?$n
 	done | zc >actual &&
-	diff -u expect actual
+	test_cmp expect actual
 
 '