dont reuse etag for folders marked explicitly unscanned

[icewind1991]

Speculated issue:

• An external smb storage is setup where the folder mtime doesn't change when new files get created
• files_external:notify is running marking the parent folder of any created file with a size of -1
• the background scanner triggers for the parent folder, finding the new file
• because the mtime of the folder didn't change, the old etag for the parent folder is reused
• the sync client doesn't pickup the change because the etag of the parent folder didn't change.

A use case that might trigger this is an external smb storage where the underlying fs doesn't change folder mtime as we expect but "files_external:notify" is being used to process external file changes.

I was able to reproduce this with manual touching to "unchange" the folder mtime a few times but then something changed and I can't anymore...

[icewind1991]

Fix confirmed working by customer

[icewind1991]

/backport to stable27

[icewind1991]

/backport to stable26

[come-nc]

@icewind1991 backport to stable28 as well no?

[icewind1991]

/backport to stable28