gitlab-ci: override git strategy / depth

In order to properly calculate what aports have been changed, we need
proper history being present. A shallow clone interferese with that.

Instead of having to override these settings on each project (forks of
aports), we can override these settings in the CI jobs description
instead.

.gitlab-ci.yml

@@ -2,6 +2,10 @@ stages:
   - lint
   - build
 
+variables:
+  GIT_STRATEGY: fetch
+  GIT_DEPTH: "0"
+
 # Make sure master points to the correct upstream commit
 before_script:
   - >