Set diff base when viewing file versions with Fugitive

When you look at previous versions of a file, e.g. with Fugitive's
:0Gclog, gitgutter will set the diff base to the current version's
parent so that diff markers appear as expected.

Thannks to @rcreasi for the idea and initial implementation.

autoload/gitgutter/utility.vim

@@ -190,8 +190,20 @@ function! s:restore_shell() abort
   endif
 endfunction
 
+function! gitgutter#utility#set_diff_base_if_fugitive(bufnr)
+  let p = resolve(expand('#'.a:bufnr.':p'))
+  let ml = matchlist(p, '\v^fugitive:/.*/(\x{40,})/')
+  if !empty(ml) && !empty(ml[1])
+    let g:gitgutter_diff_base = ml[1].'^'
+  endif
+endfunction
+
 function! s:abs_path(bufnr, shellesc)
   let p = resolve(expand('#'.a:bufnr.':p'))
+
+  " Remove extra parts from fugitive's filepaths
+  let p = substitute(substitute(p, '^fugitive:', '', ''), '\v\.git/\x{40,}/', '', '')
+
   return a:shellesc ? gitgutter#utility#shellescape(p) : p
 endfunction