ci: make the whitespace checker more robust

ci: make the whitespace checker more robust

In 32c83afc2c69 (ci: github action - add check for whitespace errors,
2020-09-22), we introduced a GitHub workflow that automatically checks
Pull Requests for whitespace problems.

However, when affected lines contain one or more double quote
characters, this workflow failed to attach the informative comment
because the Javascript snippet incorrectly interpreted these quotes
instead of using the `git log` output as-is.

Let's fix that.

While at it, let's `await` the result of the `createComment()` function.

Finally, we enclose the log in the comment with ```...``` to avoid
having the diff marker be misinterpreted as an enumeration bullet.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>