Uses Lfs::FileModificationHandler to coordinate LFS detection, creation of LfsObject, etc
Caveats:
1. This isn't used by the multi-file editor / Web IDE
2. This isn't used on rename. We'd need to be able to download LFS files
and add them to the commit if they no longer match so not as simple.
3. We only check the root .gitattributes file, so this should be improved
to correctly check for nested .gitattributes files in subfolders.
