Use GIT_DEPTH for builds

What does this MR do?

Enables experimental feature to use shallow cloning.

Why was this MR needed?

To speed up the builds and reduce the pressure on NFS servers. This should save us between 30s to 1m of the time of each build.

More information

GIT_DEPTH: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/188

Problems

• Too small value for GIT_DEPTH can make it impossible to retry old changes. You will see unresolved reference in build log. We should then reconsider changing GIT_DEPTH to higher value
• Mechanism that rely on git describe may not work correctly when GIT_DEPTH is set. This will happen, because only part of the git history is present on the build machine

Requirements

GitLab Runner 1.3.0. Currently all our internal runners use beta release or 1.3.0 with support for GIT_DEPTH.

@pcarranza Please decide when to merge that. We should start monitoring to see an impact on our NFS servers.

cc @jacobvosmaer-gitlab @pcarranza

.gitlab-ci.yml

@@ -18,6 +18,7 @@ variables:
   SIMPLECOV: "true"
   USE_DB: "true"
   USE_BUNDLE_INSTALL: "true"
+  GIT_DEPTH: "20"
 before_script:
   - source ./scripts/prepare_build.sh