DirCacheCheckout: load WorkingTreeOptions only once

DirCacheCheckout: load WorkingTreeOptions only once

Previous code loaded the WorkingTreeOptions afresh for every single
file being checked out. This checked the git config (all three files,
repo, user and system config) for having been modified every time.

These checks can be costly, for instance on Windows, or if one of the
three config files is not on a local disk, or on an otherwise slow
storage.

Improve this by loading the options and thus checking the git config
only once before the checkout.

Bug: 579715
Change-Id: I21cd5a808f9d90b5ca2d022f91f0eeb8ca26091c
Signed-off-by: Thomas Wolf <twolf@apache.org>