UploadPack: Avoid parsing want list on clone

UploadPack: Avoid parsing want list on clone

If a client wants to perform a clone of the repository, it sends
wants, but no haves.  There is no point in parsing the want list
within UploadPack, as there won't be a common merge base search.
Instead just defer the parsing to PackWriter, which will do its
own parsing and object enumeration.

If the client does have a "have" set, defer parsing of the want list
until the have list is also parsed, and parse them together in a
single batch queue.  This lets the underlying storage system use a
larger lookup batch if there is significant latency involved when
resolving an ObjectId to a RevObject.

Change-Id: I9c30d34f8e344da05c8a2c041a6dc181d8e8bc19
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>