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

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.

This adds some hooks into the filesystem iterator so that the
workdir iterator can just become a wrapper around it.  Then we
remove most of the workdir iterator code and just have it augment
the filesystem iterator with skipping .git entries, updating the
ignore stack, and checking for submodules.

This adds a new variant iterator that is a raw filesystem iterator
for scanning directories from a root.  There is still more work to
do to blend this with the working directory iterator.

Return error for empty name/email

use a longer string for dummy data in test to avoid conflicting w/ index

Allow access to the cached peel data in packed-refs

checkout: use cache when possible to determine if workdir item is dirty

Add `git_repository_open_bare`

If the on-disk file has been staged (it's stat data matches the stat data
in the cache) then we need not hash the file to determine whether it
differs from the checkout target; instead we can simply use the oid in
the index.

This prevents recomputing a file's hash unnecessarily, prevents loading
the file (when filtering) and prevents edge cases where filters suggest
that a file is dirty immediately after git writes the file.

Update link to Perl bindings