Fix ReceivePack connectivity validation with alternates
If a repository has an alternate object database, the alternate has
its references advertised as ".have" lines, which permits the client
to use these as delta base candidates when generating the pack. If
setCheckReferencedObjectsAreReachable(true) is used, these additional
have lines need to be considered in addition to the advertised refs.
Change-Id: Ie39c6696f9d3ff147ef4405cd5624f6011700ce5 Signed-off-by: Shawn O. Pearce <spearce@spearce.org>