Problem with folders with the same name, different Unicode normalization

[erikjv]

The client behaves badly when it encounters two folders in the same parent folder which (seemingly) have the same name, but a different normalization (NFC vs. NFD). For example, a folder with two subfolders named "Fälder" (NFC) and "Fälder" (NFD), will end up synced to the client as one single folder, with their contents merged.

Worse:

• on the next sync it will discover that there are new files in that folder, and upload them
• the folder that doesn't exist locally is deleted on the remote

Fix: the client should detect this normalization collision when running the discovery on the remote, and refuse to sync one of the two folders. Additionally, an error should be shown explaining the situation to the end-user.