Avoid redownload by client of locally existing files (eg copied by rsync)

### Expected behaviour
Tell us what should happen
Files that have been copied via rsync into a clients local nextcloud folder are redownloaded by the nextcloud client altough this should be avoided.
### Actual behaviour
I copied a testfile into a nexclouds client subfolder that I disabled for syncing in the nextcloud client before. Then I reenabled syncing again for this subfolder and the file was downloaded again from the server instead of using the local identical copy.

### Steps to reproduce
1. Take a file from your nextcloud server and copy it outside from nextcloud to a client into the nextcloud local folder into the correct directory
2. Watch what nextcloud client will do when you enable syncing for this directory
3. You will see it will redownload the file overwriting the identical existing one (the one that has been copied "manually" eg via rsync.

### Client configuration
Client version: 2.5.1

Operating system:
Linux, OpenSuE Leap 15.1
OS language: German

Qt version used by client package (Linux only, see also Settings dialog):
Built from Git revision b37cbe using Qt 5.9.7, OpenSSL 1.1.0i-fips 14 Aug 2018
Client package (From Nextcloud or distro) (Linux only):
From distro
Installation path of client:
/usr/bin/nextcloud

### Server configuration

Nextcloud version: 16.0.4

Storage backend (external storage):
Disk
### Logs

I found issue [#3422](https://github.com/owncloud/client/issues/3422) from 2017, where this problem was already discussed. However I did not find a solution up to now if there is any.

Personally what I want to do is what I described here:

[avoid-complete-nextcloud-resync-to-client-if-data-are-already-on-the-client-via-rsync](https://help.nextcloud.com/u/rainer042/activity/topics)

Thanks in advance
Rainer