chainlint.pl: complain about loops lacking explicit failure handling

Shell `for` and `while` loops do not terminate automatically just
because a command fails within the loop body. Instead, the loop
continues to iterate and eventually returns the exit status of the final
command of the final iteration, which may not be the command which
failed, thus it is possible for failures to go undetected. Consequently,
it is important for test authors to explicitly handle failure within the
loop body by terminating the loop manually upon failure. This can be
done by returning a non-zero exit code from within the loop body
(i.e. `|| return 1`) or exiting (i.e. `|| exit 1`) if the loop is within
a subshell, or by manually checking `$?` and taking some appropriate
action. Therefore, add logic to detect and complain about loops which
lack explicit `return` or `exit`, or `$?` check.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/chainlint/nested-loop-detect-failure.test

@@ -0,0 +1,35 @@
+# LINT: neither loop handles failure explicitly with "|| return 1"
+for i in 0 1 2 3 4 5 6 7 8 9;
+do
+	for j in 0 1 2 3 4 5 6 7 8 9;
+	do
+		echo "$i$j" >"path$i$j"
+	done
+done &&
+
+# LINT: inner loop handles failure explicitly with "|| return 1"
+for i in 0 1 2 3 4 5 6 7 8 9;
+do
+	for j in 0 1 2 3 4 5 6 7 8 9;
+	do
+		echo "$i$j" >"path$i$j" || return 1
+	done
+done &&
+
+# LINT: outer loop handles failure explicitly with "|| return 1"
+for i in 0 1 2 3 4 5 6 7 8 9;
+do
+	for j in 0 1 2 3 4 5 6 7 8 9;
+	do
+		echo "$i$j" >"path$i$j"
+	done || return 1
+done &&
+
+# LINT: inner & outer loops handles failure explicitly with "|| return 1"
+for i in 0 1 2 3 4 5 6 7 8 9;
+do
+	for j in 0 1 2 3 4 5 6 7 8 9;
+	do
+		echo "$i$j" >"path$i$j" || return 1
+	done || return 1
+done