remote-mediawiki tests: replace deprecated Perl construct

remote-mediawiki tests: replace deprecated Perl construct

The use of the encoding pragma has been a hard error since Perl
5.18 (released in 2013).

What this script really wanted to do was to decode @ARGV and write out
some files with the UTF-8 PerlIO layer. Let's just do that explicitly
instead.

This explicitly does not retain the previous UTF-8 semantics of the
script. The "encoding" pragma had all sorts of global effects (program
text being UTF-8, stdin/stdout etc.). But the only thing that was
required was decoding @ARGV and writing out UTF-8 data, which is
currently facilitated with the "open" pragma.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>