The manifest returned by merge_local_folder_manifest is sometimes incorrectly need_sync

[vxgmichel]

The manifest returned by merge_local_folder_manifest should always be need_sync=False when its content matches the remote, which is not the case at the moment (specifically when the manifest comes from ourselves),

Here's a commit referencing this issue: 863f524

Also, add a test corresponding to this case:

• A change is made locally
• The app uploads the corresponding manifest
• The app is closed before it could acknowledge the upload
• The app start again, performing an inbound sync on the freshly uploaded manifest
• Check that the merge produced a local manifest with need_sync=False in order to avoid a new inbound sync