Improve SyntaxDiffer diffing rules

[mattwar]

Fixes #463

This change alters the diffing rules by no longer choosing to identify an insert action when look ahead discovers an identical matching node in the new tree if the same identical node is also found ahead in the old tree. Identical nodes generally occur due to lexer aggressive interning, but can occur due to tree edits too.

[dnfclas]

Hi @mattwar, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!

In order for us to evaluate and accept your PR, we ask that you sign a contribution license agreement. It's all electronic and will take just minutes. I promise there's no faxing. https://cla2.dotnetfoundation.org.

TTYL, DNFBOT;

[dnfclas]

@mattwar, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;

[gafter]

👍

[gafter]

👍