t/lib-patch-mode.sh: fix ignored exit codes

t/lib-patch-mode.sh: fix ignored exit codes

Fix code added in b319ef70a94 (Add a small patch-mode testing library,
2009-08-13) to use &&-chaining.

This avoids losing both the exit code of a "git" and the "cat"
processes.

This fixes cases where we'd have e.g. missed memory leaks under
SANITIZE=leak, this code doesn't leak now as far as I can tell, but I
discovered it while looking at leaks in related code.

For "verify_saved_head()" we could make use of "test_cmp_rev" with
some changes, but it uses "git rev-parse --verify", and this existing
test does not. I think it could safely use it, but let's avoid the
while-at-it change, and narrowly fix the exit code problem.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>