github-ci: fix link to whitespace error
When the `check-whitespace` CI job detects whitespace errors, a
formatted summary of the issue is generated. This summary contains links
to the commits and blobs responsible for the whitespace errors. The
generated links for blobs do not work and result in a 404.
Instead of using the reference name in the link, use the commit ID
directly. This fixes the broken link and also helps enable future
generalization of the script for other CI providers by removing one of
the GitHub specific CI variables used.
Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>