When parsing integers from configuration values, we use `git__strtol64`.
This is fine to do, as we always sanitize values and can thus be sure
that they'll have a terminating `NUL` byte. But as this is the last
call-site of `git__strtol64`, let's just pass in the length explicitly
by calling `strlen` on the value to be able to remove `git__strtol64`
altogether.

(cherry picked from commit 1a2efd10)