vim-patch:8.1.1922: in diff mode global operations can be very slow

Problem:    In diff mode global operations can be very slow.
Solution:   Do not call diff_redraw() many times, call it once when redrawing.
            And also don't update folds multiple times.
4f57eefe1e

src/nvim/diff.c

@@ -490,7 +490,8 @@ static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
   }
 
   if (tp == curtab) {
-    diff_redraw(true);
+    // Don't redraw right away, this updates the diffs, which can be slow.
+    need_diff_redraw = true;
 
     // Need to recompute the scroll binding, may remove or add filler
     // lines (e.g., when adding lines above w_topline). But it's slow when
@@ -634,8 +635,9 @@ static int diff_check_sanity(tabpage_T *tp, diff_T *dp)
 /// Mark all diff buffers in the current tab page for redraw.
 ///
 /// @param dofold Also recompute the folds
-static void diff_redraw(int dofold)
+void diff_redraw(bool dofold)
 {
+  need_diff_redraw = false;
   FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
     if (!wp->w_p_diff) {
       continue;

src/nvim/fold.c

@@ -771,6 +771,11 @@ void foldUpdate(win_T *wp, linenr_T top, linenr_T bot)
     return;
   }
 
+  if (need_diff_redraw) {
+    // will update later
+    return;
+  }
+
   // Mark all folds from top to bot as maybe-small.
   fold_T *fp;
   (void)foldFind(&wp->w_folds, top, &fp);

src/nvim/globals.h

@@ -744,6 +744,7 @@ EXTERN int maptick INIT(= 0);            // tick for each non-mapped char
 EXTERN int must_redraw INIT(= 0);           // type of redraw necessary
 EXTERN bool skip_redraw INIT(= false);      // skip redraw once
 EXTERN bool do_redraw INIT(= false);        // extra redraw once
+EXTERN bool need_diff_redraw INIT(= false);  // need to call diff_redraw()
 EXTERN bool must_redraw_pum INIT(= false);  // redraw pum. NB: must_redraw
                                             // should also be set.
 

src/nvim/screen.c

@@ -286,6 +286,11 @@ int update_screen(int type)
     return FAIL;
   }
 
+  // May have postponed updating diffs.
+  if (need_diff_redraw) {
+    diff_redraw(true);
+  }
+
   if (must_redraw) {
     if (type < must_redraw)         /* use maximal type */
       type = must_redraw;