remote: fix tear down of struct remote

remote: fix tear down of struct remote

The remote_clear() function failed to free the remote->push and
remote->fetch refspec fields.

This should be caught by the leak sanitizer. However, for callers which
use ``the_repository``, the values never go out of scope and the
sanitizer doesn't complain.

A future change is going to add a caller of read_config() for a
submodule repository structure, which would result in the leak sanitizer
complaining.

Fix remote_clear(), updating it to properly call refspec_clear() for
both the push and fetch members.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>