When fetching into a repository which has symbolic references via the "local" transport we run into an assert. The assert is being triggered while we negotiate the packfile between the two repositories. When hiding known revisions from the packbuilder revwalk, we unconditionally hide all references of the local refdb. In case one of these references is a symbolic reference, though, this means we're trying to hide a `NULL` OID, which triggers the assert. Fix the issue by only hiding OID references from the revwalk. Add a test to catch this issue in the future.
When a refspec contains no rhs and thus won't cause an explicit update, we skip all the logic, but that means that we don't update FETCH_HEAD with it, which is what the implicit rhs is. Add another bit of logic which puts those remote heads in the list of updates so we put them into FETCH_HEAD.
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.