ReceivePack: clear advertised .haves if application changes refs

ReceivePack: clear advertised .haves if application changes refs

An application can choose to invoke setAdvertisedRefs multiple times,
for example several AdvertiseRefsHook installed in a chain. Each of
these invocations populates the advertisedHaves collection with the
unique set of ObjectIds.

This can lead to a server over-advertising with ".have" lines if the
first hook pushes in a lot of references, and the second hook filters
this to a subset.  ReceivePack will advertise the unique objects from
the first hook using ".have" lines, which may lead to a huge
advertisement sent to the client.

This can also contribute to a very slow connectivity check after the
pack is parsed as ReceivePack calls markUninteresting on every commit
in advertisedHaves.  This may require expanding a lot of subtrees to
mark all trees as uninteresting as well.  On a very big repository
this can lead to a many-second stall.

Clear the advertisedHaves collection any time the refs are updated.
Add a test to verify the correct set of objects was sent.

Change-Id: I97f6998d0597251444a2e846a3ea1f461bae96f9