UploadPack: configure RequestPolicy with TransportConfig

UploadPack: configure RequestPolicy with TransportConfig

C git 1.8.2 supports setting the equivalent of RequestPolicy.TIP with
uploadpack.allowtipsha1. Parse this into TransportConfig and use it
from UploadPack. An explicitly set RequestPolicy overrides the config,
and the policy may still be upgraded on a unidirectional connection to
avoid races.

Defer figuring out the effective RequestPolicy to later in the
process. This is a minor semantic change to fix a bug: previously,
calling setRequestPolicy(ADVERTISED) _after_ calling
setBiDirectionalPipe(true) would have reintroduced the race condition
otherwise fixed by 01888db892aa9590862d886c01f3b293140db153.

Change-Id: I264e028a76574434cecb34904d9f5944b290df78