Updates Files::MultiService for the commits API which is in turn used by the
multi-file upload web UI
Ensures that files which should be in LFS are transformed into LFS pointers
Uses Lfs::Transformer which then links LfsObjectProjects on success
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.
