git-remote-mediawiki: allow page names with a ':'

git-remote-mediawiki: allow page names with a ':'

Traditionnally, pages named Foo:Bar are page 'Bar' in namespace 'Foo'.
However, it is also possible to call a page Foo:Bar if 'Foo' is not a
namespace. In this case, the actual name of the page is 'Foo:Bar', in the
main namespace. Since we can't tell with only the filename, query the
wiki for a namespace 'Foo' in these cases, but deal with the case where
no such namespace is found.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>