Breaks in a test assertion's && chain can potentially hide failures
from earlier commands in the chain.
'unset' returns non-zero status when the variable passed was already unset
on some shells; we need to change these instances to 'sane_unset'.
Helped-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Acked-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>