remote-mediawiki: support fetching from (Main) namespace

remote-mediawiki: support fetching from (Main) namespace

When we specify a list of namespaces to fetch from, by default the MW
API will not fetch from the default namespace, refered to as "(Main)"
in the documentation:

https://www.mediawiki.org/wiki/Manual:Namespace#Built-in_namespaces

I haven't found a way to address that "(Main)" namespace when getting
the namespace ids: indeed, when listing namespaces, there is no
"canonical" field for the main namespace, although there is a "*"
field that is set to "" (empty). So in theory, we could specify the
empty namespace to get the main namespace, but that would make
specifying namespaces harder for the user: we would need to teach
users about the "empty" default namespace. It would also make the code
more complicated: we'd need to parse quotes in the configuration.

So we simply override the query here and allow the user to specify
"(Main)" since that is the publicly documented name.

Signed-off-by: Antoine Beaupré <anarcat@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>