Gthumb has gthumb-2-10 branch under gnome-extras

[Priit Laes <amd store20 com>]

Hey!

I don't know whether this is an oversight, but when selecting
gnome-extras, gthumb's branch has been set as gthumb-2-10.

There's been a lot of work going on in master branch for a while, so I
guess it would be nice to change it back to master.

Cheers,
Priit Laes
Estonian l10n Team ;)