ci: disallow directional formatting

ci: disallow directional formatting

As described in https://trojansource.codes/trojan-source.pdf, it is
possible to abuse directional formatting (a feature of Unicode) to
deceive human readers into interpreting code differently from compilers.

For example, an "if ()" expression could be enclosed in a comment, but
rendered as if it was outside of that comment. In effect, this could
fool a reviewer into misinterpreting the code flow as benign when it is
not.

It is highly unlikely that Git's source code wants to contain such
directional formatting in the first place, so let's just disallow it.

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