UploadPack: Use request instead of field for depth

UploadPack: Use request instead of field for depth

One more step in removing state from UploadPack, using the request
object instead.

Unfortunately, hooks get from UploadPack information about the current
request. Changing the hooks to receive the request is a public API
change, so at the moment lets keep a reference to the current request.

This kills half the benefit of using a request object vs fields, but
at least we still get better modularity.

Change-Id: I86803d876a621b727c66ee73f2880c93190629e9
Signed-off-by: Ivan Frade <ifrade@google.com>