git-remote-mediawiki: import "File:" attachments

git-remote-mediawiki: import "File:" attachments

Add the symmetrical feature to the "File:" export support in the previous
patch. Download files from the wiki as needed, and feed them into the
fast-import stream. Import both the file itself, and the corresponding
description page.

Signed-off-by: Pavel Volek <Pavel.Volek@ensimag.imag.fr>
Signed-off-by: NGUYEN Kim Thuat <Kim-Thuat.Nguyen@ensimag.imag.fr>
Signed-off-by: ROUCHER IGLESIAS Javier <roucherj@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>