--commit should detect readding packages that were pkgmoved

[mgorny]

It would be nice if pkgcheck scan --commit would report if the newly added package matches an existing move entry, e.g. if I add app-text/pytextile I should get a warning like "adding a new package that matches an existing package move entry (moved to dev-python/textile)".

[mgorny]

Thinking about it more, perhaps it's entirely redundant to #562 since the previously-moved packages are going to exist in history. I suppose the only case this could do something is when you don't have full history.