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

Add filesystem iterator variant

When attempting to create a workdir iterator for a bare repo,
don't leak the iterator structure.

Refactors the helper function that builds a directory hierarchy
and then made use of it to try more variations on filesystem
iterator tests.

Renamed the callback functions and made some minor rearrangements
to clean up the flow of some code.