t/README: correct an exception when breaking a && chain in tests
t/README: correct an exception when breaking a && chain in tests

The correct advice should have been taken from c289c31 (t/t7006: ignore
return status of shell's unset builtin, 2010-06-02).  A real-life issue
we experienced was with "unset", not with "export" (exporting an
unset variable may have similar portability issues, though).

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git