t6200: avoid path mangling issue on Windows

MSYS bash interprets the slash in the argument core.commentchar="/"
as root directory and mangles it into a Windows style path. Use a
different core.commentchar to dodge the issue.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>