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.
When commiting multiple files we're now checking if any of those files
has been modified by another commit and we're rejecting the new commit
in this case.
Currently when a user performs an update of a file through the UI and there
has already been a change committed to the file the previous commits will be
overwritten without a check to see if the file has been changed.
This commit uses the last commit sha at the time the user starts editing the
file and compares it with the current sha of the file being edited to ensure
they are the same before committing the file. If the shas do not match we
throw an exception preventing the commit from the commit from occurring.
Fixes#5857
