(core) hide long sequences of unchanged rows in diffs

Summary:
It can be hard to find changes, even when highlighted, in a table with many rows.  This diff replaces long sequences of unchanged rows with a row containing "..."s.

With daff, I found that it is important to do this for sequences of unchanged columns also, but not tackling that yet.

Test Plan: added test

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2666

app/client/components/viewCommon.css

@@ -75,6 +75,11 @@
   background-color: unset;
 }
 
+.field_clip.field-error-S {
+  color: #aaa;
+  background-color: unset;
+}
+
 /* Insert a zero-width space into each cell, to size cells to at least one line of text. */
 .field_clip:empty::before { content: '\200B'; }