Introduce ProtocolV2Hook

Introduce ProtocolV2Hook

In Git protocol v2, UploadPack and ReceivePack have the same
capabilities and can process any protocol v2 request. For example, a
client can sent a "fetch" command to the "/git-receive-pack" endpoint.

This makes it difficult for existing hook interfaces. For example,
PreUploadHook takes UploadPack, but a "fetch" command may be received by
ReceivePack.

To resolve this skew, this change introduce a different hook interface
for the protocol v2. The hook takes a request that is independent to the
handlers (UploadPack, ReceivePack). Also this makes it clear what
parameters the hook is counting on, instead of keep track of the hook
using getters from UploadPack / ReceivePack.

Bug: 534847
Change-Id: I71f3266584483db1e2b2edfc1a72d0bdf1bb6041
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>