If it's not documented, it doesn't show up in the docs (and we really
should document, anyway).

Keep the reflog parameters as the last two, as they're the optional
parameters.

This addresses arrbee's concerns about wording in the conditional
reference udpate functions.

Add it under the git_reference_remove() name, letting the user pass the
repo and name, analogous to unconditional setting/creation.

Recognize when the reference has changed since we loaded it.

Bring the race detection goodness to symbolic references as well.