test-lib-functions: detect test_when_finished in subshell

test_when_finished does nothing in a subshell because the change to
test_cleanup does not affect the parent.

There is no POSIX way to detect that we are in a subshell ($$ and $PPID
are specified to remain unchanged), but we can detect it on Bash and
fall back to ignoring the bug on other shells.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -736,6 +736,11 @@ test_seq () {
 # what went wrong.
 
 test_when_finished () {
+	# We cannot detect when we are in a subshell in general, but by
+	# doing so on Bash is better than nothing (the test will
+	# silently pass on other shells).
+	test "${BASH_SUBSHELL-0}" = 0 ||
+	error "bug in test script: test_when_finished does nothing in a subshell"
 	test_cleanup="{ $*
 		} && (exit \"\$eval_ret\"); eval_ret=\$?; $test_cleanup"
 }