Fix status diffs broken by missing sub attribute

Fixes https://github.com/tpope/vim-fugitive/issues/1367

autoload/fugitive.vim

@@ -2531,6 +2531,7 @@ function! s:StageInfo(...) abort
         \ 'paths': map(reverse(split(text, ' -> ')), 's:Tree() . "/" . v:val'),
         \ 'commit': matchstr(getline(lnum), '^\%(\%(\x\x\x\)\@!\l\+\s\+\)\=\zs[0-9a-f]\{4,\}\ze '),
         \ 'status': matchstr(getline(lnum), '^[A-Z?]\ze \|^\%(\x\x\x\)\@!\l\+\ze [0-9a-f]'),
+        \ 'sub': get(get(get(b:fugitive_files, section, {}), text, {}), 'sub', ''),
         \ 'index': index}
 endfunction