xdiff: avoid using the comma operator unnecessarily

The comma operator is a somewhat obscure C feature that is often used by
mistake and can even cause unintentional code flow. While the code in
this patch used the comma operator intentionally (to avoid curly
brackets around two statements, each, that want to be guarded by a
condition), it is better to surround it with curly brackets and to use a
semicolon instead.

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>