Don't touch file system if the first change is AddFile

[Xanewok]

Originally created by @matklad (rust-dev-tools/rls-vfs#11):

When the first change to a file is AddFile we could avoid touching the disk for better performance.

If the first change is not AddFile, we might want to log a warning, because we can't 100% guarantee consistency then. Though, this is unlikely to cause major problems in practice.