Does not generate correct .gitattributes entry for filenames with spaces

git-lfs-migrate seems to generate a .gitattributes file with spaces in the file names left intact, causing "not a valid attribute name" errors upon cloning. Apparently it should replace spaces with `[:space:]` instead. See https://github.com/git-lfs/git-lfs/issues/298 and https://github.com/git-lfs/git-lfs/pull/327.