Sync endpoint always sets up the file sytem

The sync endpoint actually always sets up the filesystem for the user due to the current way we check if the file has changed outside:

We mainly need to obtain the mtime and etag of the file to detect if it was changed or if it should be autosaved and ideally we only need to go to the filesystem layer in those cases. However I currently don't see a proper way of only obtaining those details without querying the filecache directly, which we could do since we have the file id. Alternatively we can probably introduce our own state handling by hooking into the postWrite hook and basically marking text documents as changed outside then.

@rullzer Do you see any other way? I'd probably prefer just querying the filecache directly from a implementation perspective but the hook feels a bit cleaner.

Start at the sync endpoint and trace how it checks whether a file changed outside the editor, including the filesystem and filecache layers. Compare obtaining the file's mtime and etag through the filecache with tracking changes through the postWrite hook; done means unchanged files no longer set up the filesystem while changed or autosaved files still work.