t4000-t4999: detect and signal failure within loop

Failures within `for` and `while` loops can go unnoticed if not detected
and signaled manually since the loop itself does not abort when a
contained command fails, nor will a failure necessarily be detected when
the loop finishes since the loop returns the exit code of the last
command it ran on the final iteration, which may not be the command
which failed. Therefore, detect and signal failures manually within
loops using the idiom `|| return 1` (or `|| exit 1` within subshells).

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4024-diff-optimize-common.sh

@@ -148,7 +148,7 @@ test_expect_success 'diff -U0' '
 
 	for n in $sample
 	do
-		git diff -U0 file-?$n
+		git diff -U0 file-?$n || return 1
 	done | zc >actual &&
 	test_cmp expect actual