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.

We want to use the "checkout: moving from ..." message in order to let
git know when a change of branch has happened. Make the convenience
functions for this goal write this message.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

If the remote does not advertise HEAD, then it is unborn and we cannot
checkout that branch. Handle it the same way as an empty repo.

Add tests for the case when there are no branches on the remote and when
HEAD is detached but has the id of a non-branch. In both of these cases,
we should return ENOTFOUND.

Assert what we already do, so as to notice changes.

If the remote supports the symref protocol extension, then we return
that, otherwise we guess with git's rules.