When we moved from acting on the instance to acting on the
configuration, we dropped the validation of the passed refspec, which
can lead to writing an invalid refspec to the configuration. Bring that
validation back.

An anonymous remote is not configured and cannot therefore have
configured refspecs. Remove the parameter which adds this from the
constructor.

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

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.

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.