Teach UploadPack basic "fetch" command

Teach UploadPack basic "fetch" command

Add basic support for the "fetch" command in the fetch-pack/upload-pack
protocol v2. This patch teaches "have" and "done".

The protocol specification (Documentation/technical/protocol-v2.txt in
the Git project) states:

    want <oid>
	Indicates to the server an object which the client wants to
	retrieve.  Wants can be anything and are not limited to
	advertised objects.

It is unspecified whether the server should respect the
uploadpack.allowtipsha1inwant option etc. when serving packfiles. This
patch is conservative in that the server respects them.

Change-Id: I3dbec172239712ef9286a15b8407e86b87ea7863
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>