Git.pm: revert _temp_cache use of temp_is_locked

When the temp_is_locked function was introduced, there was
a desire to make _temp_cache use it.  Unfortunately due to the
various tests and logic flow involved changing the _temp_cache
function to use the new temp_is_locked function is problematic
as _temp_cache needs a slightly different test than is provided
by the temp_is_locked function.

This change reverts use of temp_is_locked in the _temp_cache
function and restores the original code that existed there
before the temp_is_locked function was added.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>