Fix scrollbar sizes for diff mode

Scrollbars were not properly sized since folds were not retained in the
window workspace used for sizing calculations. The problem was most
noticeable with many lines and few differences.

Author: dstein64

lua/scrollview.lua

@@ -547,7 +547,11 @@ local with_win_workspace = function(winid, fun)
       -- Don't include the workspace window in a diff session. If included,
       -- closing it could end the diff session (e.g., when there is one other
       -- window in the session). Issue #57.
-      set_window_option(workspace_winid, 'diff', false)
+      if api.nvim_win_get_option(0, 'foldmethod') == 'diff' then
+        -- First set foldmethod to manual so that folds are retained.
+        set_window_option(workspace_winid, 'foldmethod', 'manual')
+        set_window_option(workspace_winid, 'diff', false)
+      end
     end
   end
   local success, result = pcall(function()