fix: correctly detect edit blocks in diff-fenced mode

[zjy1412]

Fixed these issues:
3785
3811
3793

The root of the problem was that the match for <<<<<<< SEARCH only matched the line below ```. However, diff-fenced is two lines below ```. Therefore, by matching one more line, it solves the issue of edits being incorrectly identified as commands.

[CLAassistant]

All committers have signed the CLA.

[iacore]

Who can merge this :)

[paul-gauthier]

Thanks @zjy1412 !