Collision detection for group folder target

[rullzer]

With the revert in #647 we have the following issue again:

1. Create a folder 'foo' for user 'bar'
2. Add some files
3. Now an admin sets up a group folder 'foo'
4. 'bar' loses access to their original 'foo'

@icewind1991 any idea how to solve this?

[PVince81]

also see #1246

[ChristophWurst]

Potential fix at #1239

[KodoGit]

Looks like #1246 leads here. The problem still exists with NC 21.0.4 docker (nextcloud:21-apache) with GF 9.0.3
Just updated from 20.

[fschrempf]

Looks like #1246 leads here. The problem still exists with NC 21.0.4 docker (nextcloud:21-apache) with GF 9.0.3

#1246 and #681 are about collisions between two groupfolders with the same name. This issue is still unresolved.
Whereas this issue seems to be about name collisions between a normal directory and a groupfolder. This issue is resolved.