Only diff against parent commit in Fugitive revision buffers

Fixes #695.

autoload/gitgutter.vim

@@ -33,8 +33,6 @@ function! gitgutter#process_buffer(bufnr, force) abort
       return
     endif
 
-    call gitgutter#utility#set_diff_base_if_fugitive(a:bufnr)
-
     if a:force || s:has_fresh_changes(a:bufnr)
 
       let diff = 'NOT SET'

autoload/gitgutter/diff.vim

@@ -120,7 +120,7 @@ function! gitgutter#diff#run_diff(bufnr, from, preserve_full_diff) abort
     endif
 
     " Write file from index to temporary file.
-    let index_name = g:gitgutter_diff_base.':'.gitgutter#utility#repo_path(a:bufnr, 1)
+    let index_name = gitgutter#utility#get_diff_base(a:bufnr).':'.gitgutter#utility#repo_path(a:bufnr, 1)
     let cmd .= g:gitgutter_git_executable.' '.g:gitgutter_git_args.' --no-pager show '.index_name.' > '.from_file.' && '
 
   elseif a:from ==# 'working_tree'

autoload/gitgutter/utility.vim

@@ -184,12 +184,13 @@ function! s:restore_shell() abort
   endif
 endfunction
 
-function! gitgutter#utility#set_diff_base_if_fugitive(bufnr)
+function! gitgutter#utility#get_diff_base(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].'^'
+    return ml[1].'^'
   endif
+  return g:gitgutter_diff_base
 endfunction
 
 function! s:abs_path(bufnr, shellesc)