Permit disabling birthday attack checks in PackParser

Permit disabling birthday attack checks in PackParser

Reading a repository for millions of missing objects might be very
expensive to perform, especially if the repository is on a network
filesystem or some other costly RPC backend.  A repository owner
might choose to accept some risk in return for better performance,
so allow disabling collision checking when receiving a pack.

Currently there is no way for an end-user to disable this feature.
This is intentional, because it is generally *NOT* a good idea to
skip this check.  Instead this feature is supplied for storage
implementations to bypass the default checking logic, should they
have their own custom routines that is just as effective but can
be handled more efficiently.

Change-Id: I90c801bb40e86412209de0c43e294a28f6a767a5
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>