Add fetch support to ProtocolV2Hook

Add fetch support to ProtocolV2Hook

This makes it symmetrical with ls-refs operation and gives the
instantiator of UploadPack the chance to run some code after parsing
the protocol and before any actual work for the fetch starts.

Request and Builder methods keep the naming in the original code to
make this change just about request encapsulation and hook invocation.
They are package-private for now to allow further improvements.

Change-Id: I5ad585c914d3a5f23b11c8251803faa224beffb4
Signed-off-by: Ivan Frade <ifrade@google.com>