Instead of going through a special entry in the chain, let's pass it as
an output parameter.

Bring back the use of the delta base cache for unpacking objects. When
generating the delta chain, we stop when we find a delta base in the
pack's cache and use that as the starting point.

We currently make use of recursive function calls to unpack an object,
resolving the deltas as we come back down the chain. This means that we
have unbounded stack growth as we look up objects in a pack.

This is now done in two steps: first we figure out what the dependency
chain is by looking up the delta bases until we reach a non-delta
object, pushing the information we need onto a stack and then we pop
from that stack and apply the deltas until there are no more left.

This version of the code does not make use of the delta base cache so it
is slower than what's in the mainline. A later commit will reintroduce
it.

Rename git_packfile_check to git_packfile_alloc since it is now
being used more in that capacity.  Fix the various places that use
it.  Consolidate some repeated code in odb_pack.c related to the
allocation of a new pack_backend.

This builds on the earlier thread safety work to make it so that
setting the odb, index, refdb, or config for a repository is done
in a threadsafe manner with minimized locking time.  This is done
by adding a lock to the repository object and using it to guard
the assignment of the above listed pointers.  The lock is only
held to assign the pointer value.

This also contains some minor fixes to the other work with pack
files to reduce the time that locks are being held to and fix an
apparently memory leak.

When I was writing threading tests for the new cache, the main
error I kept running into was a pack file having it's content
unmapped underneath the running thread.  This adds a lock around
the routines that map and unmap the pack data so that threads can
effectively reload the data when they need it.

This also required reworking the error handling paths in a couple
places in the code which I tried to make consistent.