Some of our tests try to make sure Git behaves sensibly in a
read-only directory, by dropping 'w' permission bit before doing a
test and then restoring it after it is done.  The latter is needed
for the test framework to clean after itself without leaving a
leftover directory that cannot be removed.

Ancient parts of tests however arrange the above with

	chmod a-w . &&
	... do the test ...
	status=$?
	chmod 775 .
	(exit $status)

which obviously would not work if the test somehow dies before it
has the chance to do "chmod 775".  Rewrite them by following a more
robust pattern recently written tests use, which is

	test_when_finished "chmod 775 ." &&
	chmod a-w . &&
	... do the test ...

Signed-off-by: Junio C Hamano <gitster@pobox.com>