New local file when first sync can be optimized

When sync a Folder with remote content where local files already exists, find a way to prevent upload file on existing (and same) remote file. Like a command which compare remote and local file by checksum (need Tracim feature).

Start by tracing the folder-sync command and how it handles local files when remote content already exists; review the Tracim synchronization behavior mentioned in the issue. The work is done when an initial sync can identify an identical remote file and avoid uploading it, with the comparison behavior verified.

Written by the indexing model from the issue text.