Action does not update or remove the comment when PR has zero changes

[madhusudhand]

When PR is updated with zero theme changes it fails to update the comment.

Steps:

• Create a PR with a change to a theme and another file such as readme.md
• Action adds a comment with one preview link
• Now revert the theme change and update the PR (now PR has only change to readme.md)
• Observe that action fails to update the comment.

Expectation:

Action should remove the comment when there are no theme changes, and if there is a previous comment in the PR.
Alternatively, it may also update the comment to remove all links and just say no theme changes

[vcanales]

Thanks for the report, I've updated the action with a couple of improvements. #22

• Better diffing: we will no longer see irrelevant changes being displayed because we now compare the branch HEAD with the base ref from which the branch was created instead of the base branch.
• Added a delete step that removes the comment if no theme changes are detected.