detached-stash: work around git rev-parse failure to detect bad log refs

This commit is required because git rev-parse in 1.7.2 does not correctly
indicate invalid log references using a non-zero status code.

We use a proxy for the condition (non-empty error output) as
a substitute. This commit can be reverted when, and if, rev-parse
is fixed to indicate invalid log references with a status code.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>