fetch_spec and ghosts

[Andrew Bennetts]

Jelmer Vernooij wrote:
> fetch_spec.get_keys() seems to return ghost revisions that are not in
> the source repository. Should I silently be ignoring unknown revisions?

Good question!  I think that would be reasonable, it's basically what
revision_id=foo does.

> Also, it makes fetching of Subversion repositories a lot slower, since
> looking up individual revisions is quite expensive and get_keys()
> returns a large number of revisions. get_recipe() should return the set
> of head revisions but is not implemented for PendingAncestryResult.

I didn't implement get_recipe because part of the point was to avoid
calculating the third part of the recipe, the key count.  But my HPSS code
won't call get_recipe on a PendingAncestryResult, so maybe I shouldn't mind,
or maybe we should just allow None as they key count in a recipe?  I'd be
happy to accept or write a patch along these lines.

Thanks for the timely feedback!

-Andrew.