This adds create and free callback to the git_objects_table so
that more of the creation and destruction of objects can be table
driven instead of using switch statements.  This also makes the
semantics of certain object creation functions consistent so that
we can make better use of function pointers.  This also fixes a
theoretical error case where an object allocation fails and we
end up storing NULL into the cache.

This adds some basic tests for the oidmap just to make sure that
collisions, etc. are dealt with correctly.

This also adds some tests for the new caching that check if items
are inserted (or not inserted) properly into the cache, and that
the cache can hold up in a multithreaded environment without error.

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.

Add a git_cache_set_max_object_size method that does more checking
around setting the max object size.  Also add a git_cache_size to
read the number of objects currently in the cache.  This makes it
easier to write tests.

This uses the odb object accessors so we can change the internals
more easily...

Do not allow duplicate ODB backends

Revamp the refspec handling

Conflicts:
	src/odb.c

Create include/git2/sys and move backend APIs there

Add missing prototype for p_realpath().

Create a new include/git2/sys/refs.h and move the reference alloc
functions there.  Also fix some documentation issues and some
minor code cleanups.

This moves most of the refdb stuff over to the include/git2/sys
directory, with some minor shifts in function organization.

While I was making the necessary updates, I also removed the
trailing whitespace in a few files that I modified just because I
was there and it was bugging me.

Actually this renames git_commit_create_oid to
git_commit_create_from_oids and moves the API declaration to
include/git2/sys/commit.h since it is a dangerous API for general
use (because it doesn't check that the OID list items actually
refer to real objects).

This moves some of the odb_backend stuff that is related to the
internals of an odb_backend implementation into include/git2/sys.

Some of the stuff related to streaming I left in include/git2
because it seemed like it would be reasonably needed by a normal
user who wanted to stream objects into and out of the ODB.

Also, I added APIs for traversing the list of backends so that
some of the tests would not need to access ODB internals.

Moving backend implementor objects into include/git2/sys so the
APIs can be isolated from the ones that normal libgit2 users
would be likely to use.

mailmap: Coalesce some identities

It used to be separate as an attempt to make the querying easier, but
it didn't work out that way, so put all the data together.

Add git_refspec_string() as well to get the original string, which is
now stored alongside the independent parts.

Introduce git_remote_{fetch,push}_refspecs() to get a list of refspecs
from the remote and rename the refspec-adding functions to a less
silly name.

Use this instead of the vector index hacks in the tests.

A remote can have a multitude of refspecs. Up to now our git_remote's
have supported a single one for each fetch and push out of simplicity
to get something working.

Let the remotes and internal code know about multiple remotes and get
the tests passing with them.

Instead of setting a refspec, the external users can clear all and add
refspecs. This should be enough for most uses, though we're still
missing a querying function.

Adding a multivar when there are no variables with that name set
should set the variable instead of failing.

alloc doesn't take a refdb

This fixes #1487