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.

These offsets are needed for REF_DELTA objects, which encode which
object they use as a base, but not where it lies in the packfile, so
we need a list.

These objects are mostly from older packfiles, before OFS_DELTA was
widely spread. The time spent in indexing these packfiles is greatly
reduced, though remains above what git is able to do.

The indexer needs to call the packfile's free function so it takes care of
freeing the caches.

We still need to close the mwf descriptor manually so we can rename the
packfile into its final name on Windows.

This reverts commit f289f886cb81bb570bed747053d5ebf8aba6bef7, which
makes the tests fail on Windows. Revert until we can figure out a
solution.

Currently limited to 16MB (like git) and to objects up to 1MB in
size.