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.

Add a prune setting in the fetch options to allow to fall back to the
configuration (the default) or to set it on or off.

As a first step in removing the repository-saving logic, don't allow
chaning the url or push url from a remote object, but change the
configuration on the configuration immediately.

Having the setting be different from calling its actions was not a great
idea and made for the sake of the wrong convenience.

Instead of that, accept either fetch options, push options or the
callbacks when dealing with the remote. The fetch options are currently
only the callbacks, but more options will be moved from setters and
getters on the remote to the options.

This does mean passing the same struct along the different functions but
the typical use-case will only call git_remote_fetch() or
git_remote_push() and so won't notice much difference.

It can be useful for the caller to know which update commands will be
sent to the server before the packfile is pushed up. git does this via
the pre-push hook.

We don't have hooks, but as it adds introspection into what is
happening, we can add a callback which performs the same function.

We always use "update by push".