A remote's URLs are now modified according to the url.*.insteadOf
and url.*.pushInsteadOf configurations. This allows a user to
replace URL prefixes by setting the corresponding keys. E.g.
"url.foo.insteadOf = bar" would replace the prefix "bar" with the
new prefix "foo".

These are updates, same as the rest, we should call this callback. As we
are using the callback, let's make sure to skip unnecessary updates.

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.

These were left over from the culling as it's not clear which use-cases
might benefit from this. It is not clear that we want to support any
use-case which depends on changing the remote's idea of the base
refspecs rather than passing in different per-operation refspec list, so
remove these functions.

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.