Add files whitelist to change detection

[jhamon]

Problem

We only want to trigger post-merge actions on some files

Solution

Implement a DIFF_WHITELIST env variable that is used to limit the output from git diff

These git commands are using the feature that allows paths to be specified as positional arguments after --

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)