[build-bot] Fix skipping comments in ignore list

Summary:
Attempting to check the first character of a byte sequence does not make sense.
This patch fixes that check in order to correctly skip comments in the ignore list and
provides a false-positive test to ensure we do not regress.

Test Plan: pytest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8030