guard against DDOSing PsiElementFactory#createTypeByFQClassName with many expensive file scopes (IDEA-210047)

`fileScope(LightVirtualFile)` can hold hard reference to PSI and AST, and the memory snapshot contains many of those

Instead of awkwardly implementing UnparsedChanges, descendant classes just need to provide an instance of ChangesParser to the VcsChangesLazilyParsedDetails constructor.

We also need to refresh VFS after package installation

Add a conflict and check changes to parents and merged changes.

In f8a956f5 an option was introduced to get no diff for merge commits. In a457eeef is was made default by mistake, while actually combined diff if expected. For example, GitPushOperationSingleRepoTest was broken because of this change.  In oder to fix the problem a third option is added for combined diff and used by default.