New snippets get diff highlighted

[ganicke]

Problem: A new tutorial snippet shown in a later step is highlighted when ### @diffs true is set.

Description: The entire block of code is highlighted in later steps when new code is shown that is separate from the rest of the project. It seems that a new snippet shown without the rest of the project code should have it's own diff context.

Example:

RE: https://github.com/microsoft/pxt-microbit/pull/3288