Pharo repository forgets packages

[guillep]

When getting a new pharo image, it comes with a pre-calculated list of pharo packages.

When cloning a pharo repository that does not contain the image commit (for example, one's own fork), the IceWorkingCopy resolves it as an UnknownCommit. Somehow, this causes iceberg also to forget all packages, and thus their dirty state.

Moreover, if we fetch and the commit is found in one remote, then the list is changed, but changes are not calculated anymore because the UnknownCommit id and the now found commit id are the same.