Provide error messages for git_path_direach operations

We set an error if we get an error when reading, but we don't bother
setting an error message for write failing. This causes a cryptic error
to be shown to the user when the target filesystem is full.

Now that `git_path_direach` lets us specify an error message to report,
set an appropriate error message while linking.

This function deals with functions doing IO which means the amount of
errors that can happen is quit large. It does not help if it always
ovewrites the underlying error message with a less understandable
version of "something went wrong".

Instead, only use this generic message if there was no error set by the
callback.

Updated dlibgit link.

Bring Early EOF message in line with the other two

Add tests for and fix push negotiation notification

There was a copypasta error and the source and destination IDs were
reversed.

The functionality was meged without including tests, so let's add them
now.

Attributes: don't match files for folders

Use the packbuilder in local push

Show progress during packing for the local transport

odb: make the writestream's size a git_off_t

The interesting one is the notification macro, which was returning
directly on a soft-abort instead of going through the cleanup.

This is useful to send to the client while we're performing the work.

The reporting function has a force parameter which makes sure that we
do send out the message of 100% completed, even if this comes before the
next udpate window.

Set a callback for the packbuilder so we can send the sideband messages
to the caller, formatting them as git would.

This lets us see what the server (or libgit2 locally) is doing, rather
than having to stare at a non-moving screen.

index: make the entries have more accurate sizes

Get rid of libssh2 embedding

submodule: add test initialising and cloning a repo

odb: reverse the default backend priorities

A couple of tests use the wrong remote to push to. We did not notice up
to now because the local push would copy individual objects, and those
already existed, so it became a no-op.

Once we made local push create the packfile, it became noticeable that
there was a new packfile where it didn't belong.

Instead of copying each object individually, as we'd been doing, use the
packbuilder which should be faster and give us some feedback.

While performing this change, we can hook up the packbuilder's writing
to the push progress so the caller knows how far along we are.

Remove the configuration state we keep in the remote

We currently first look in the loose object dir and then in the packs
for objects. When performing operations on recent history this has a
higher likelihood of hitting, but when we deal with operations which
look further back into the past, we start spending a large amount of
time getting ENOTENT from `access`.

Reversing the priorities means that long-running operations can get to
their objects faster, as we can look at the index data we have in memory
(or rather mapped) to figure out whether we have an object, which is
faster than going out to the filesystem.

The packed backend already implements an optimistic read algorithm by
first looking at the packs we know about and only going out to disk to
referesh if the object is not found which means that in the case where
we do have the object (which will be in the majority for anything that
traverses the graph) we can avoid going to to disk entirely to determine
whether an object exists.

Operations which look at recent history may take a slight impact, but
these would be operations which look a lot less at object and thus take
less time regardless.

We're down to simply having it be a call to create_internal() so let's
simply do that. The rest of the code is just a distraction.

It has now become a no-op, so remove the function and all references to
it.

The base refspecs changing can be a cause of confusion as to what is the
current base refspec set and complicate saving the remote's
configuration.

Change `git_remote_add_{fetch,push}()` to update the configuration
instead of an instance.

This finally makes `git_remote_save()` a no-op, it will be removed in a
later commit.

This is another option which we should not be keeping in the remote, but
is specific to each particular operation.

While this will rarely be different from the default, having it in the
remote adds yet another setting it has to keep around and can affect its
behaviour. Move it to the options.

Instead of having it set in a different place from every other callback,
put it the main structure. This removes some state from the remote and
makes it behave more like clone, where the constructors are passed via
the options.