largefiles: put whole rollback-ing process into the same "wlock" scope
Before this patch, there are three distinct "wlock" scopes in
"overriderollback":
1. "localrepository.rollback" via original "rollback" command,
2. "merge.update" for reverting standin files only, and
3. "overriderollback" specific "wlock" scope (to synchronize
largefile dirstate)
But these should be executed in the same "wlock" scope for
consistency.