VIM-511 Fix buggy repeat command related to embedded backspaces.

This is to address issue VIM-511.

The existing document listener adds an extra "right" cursor motion on backspaces, but the complications of online editing make it very difficult to spot these during editing. So, instead I added in a post-filtering step that recognizes the pattern and removes if from the strokes to be repeated before they are repeated. It is kind of a hack, but it gets the majority of cases working.