View Changes: external changes desync state and trigger checks

External changes to source control are causing state desyncs which get expressed in code as multiple operations on the same items in the same UI visit. This check is in place because when only using the View Changes menu this should never happen. We need to make the View Changes data model resilient to external changes in the source control provider.

From UDN:

1. Have Editor connected to Perforce source control.
2. In your project, open a Map that uses OFPA.
3. Make an arbitrary change to an Actor and save it to force a Checkout.
4. Open the View Changes panel, hopefully the file will be listed in the default changelist.
5. In the P4 app, manually revert the file. Don't manually refresh in the View Changes panel!
6. Repeat step 3 to re-check out the file again.
7. You should observe the checkf being triggered in OnSourceControlCachedItemVisited.