patch 8.2.4941: '[ and '] marks may be wrong after undo

Problem:    '[ and '] marks may be wrong after undo.
Solution:   Adjust the '[ and '] marks if needed. (closes #10407, closes #1281)

src/testdir/test_undo.vim

@@ -756,4 +756,21 @@ func Test_redo_multibyte_in_insert_mode()
   bwipe!
 endfunc
 
+func Test_undo_mark()
+  new
+  " The undo is applied to the only line.
+  call setline(1, 'hello')
+  call feedkeys("ggyiw$p", 'xt')
+  undo
+  call assert_equal([0, 1, 1, 0], getpos("'["))
+  call assert_equal([0, 1, 1, 0], getpos("']"))
+  " The undo removes the last line.
+  call feedkeys("Goaaaa\<Esc>", 'xt')
+  call feedkeys("obbbb\<Esc>", 'xt')
+  undo
+  call assert_equal([0, 2, 1, 0], getpos("'["))
+  call assert_equal([0, 2, 1, 0], getpos("']"))
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/undo.c

@@ -2831,9 +2831,10 @@ u_undoredo(int undo)
 	if (oldsize > 0 || newsize > 0)
 	    changed_lines(top + 1, 0, bot, newsize - oldsize);
 
-	// set '[ and '] mark
+	// Set the '[ mark.
 	if (top + 1 < curbuf->b_op_start.lnum)
 	    curbuf->b_op_start.lnum = top + 1;
+	// Set the '] mark.
 	if (newsize == 0 && top + 1 > curbuf->b_op_end.lnum)
 	    curbuf->b_op_end.lnum = top + 1;
 	else if (top + newsize > curbuf->b_op_end.lnum)
@@ -2853,6 +2854,12 @@ u_undoredo(int undo)
 	newlist = uep;
     }
 
+    // Ensure the '[ and '] marks are within bounds.
+    if (curbuf->b_op_start.lnum > curbuf->b_ml.ml_line_count)
+	 curbuf->b_op_start.lnum = curbuf->b_ml.ml_line_count;
+    if (curbuf->b_op_end.lnum > curbuf->b_ml.ml_line_count)
+	 curbuf->b_op_end.lnum = curbuf->b_ml.ml_line_count;
+
     // Set the cursor to the desired position.  Check that the line is valid.
     curwin->w_cursor = new_curpos;
     check_cursor_lnum();

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4941,
 /**/
     4940,
 /**/