git-remote-mediawiki: modify strings for a better coding-style

- strings which don't need interpolation are single-quoted for more clarity and
slight gain of performance
- interpolation is preferred over concatenation in many cases, for more clarity
- variables are always used with the ${} operator inside strings
- strings including double-quotes are written with qq() so that the quotes do
not have to be escaped

Signed-off-by: Célestin Matte <celestin.matte@ensimag.fr>
Signed-off-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>