Move embedded screenshot from to-be-retired wiki.gnome.org to GitLab repository
Per https://discourse.gnome.org/t/wiki-gnome-org-retirement-feedback-wanted/17314/28
Image should probably be moved in GitLab code repository and called via https://gitlab.gnome.org/GNOME/$reponame/-/raw/$branchname/path/to/image/$imagename.png
./gitg/data/org.gnome.gitg.appdata.xml.in.in: <image>https://wiki.gnome.org/Apps/Gitg?action=AttachFile&do=get&target=history.png</image>
./gitg/data/org.gnome.gitg.appdata.xml.in.in: <image>https://wiki.gnome.org/Apps/Gitg?action=AttachFile&do=get&target=stage.png</image>
./gitg/data/org.gnome.gitg.appdata.xml.in.in: <image>https://wiki.gnome.org/Apps/Gitg?action=AttachFile&do=get&target=commit.png</image>
./gitg/data/org.gnome.gitg.appdata.xml.in.in: <image>https://wiki.gnome.org/Apps/Gitg?action=AttachFile&do=get&target=projects.png</image>