compat/regex: explicitly mark intentional use of the comma operator

The comma operator is a somewhat obscure C feature that is often used by
mistake and can even cause unintentional code flow. That is why the
`-Wcomma` option of clang was introduced: To identify unintentional uses
of the comma operator.

In the `compat/regex/` code, the comma operator is used twice, once to
avoid surrounding two conditional statements with curly brackets, the
other one to increment two counters simultaneously in a `do ... while`
condition.

The first one is replaced with a proper conditional block, surrounded by
curly brackets.

The second one would be harder to replace because the loop contains two
`continue`s. Therefore, the second one is marked as intentional by
casting the value-to-discard to `void`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>