This can be used by tools to show mesages about failing to communicate
with the server. The error message in this case will often contain the
server's error message, as far as it managed to send anything.

odb: make the writestream's size a git_off_t

index: make the entries have more accurate sizes

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.