CI: Allow use with shallow clone CI setup

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>

.gitlab-ci.yml

@@ -76,7 +76,7 @@ stages:
     # The repo tool seems to insist on having a branch checked out or
     # something like that... Without this we get errors like
     #     fatal: couldn't find remote ref refs/heads/master
-    - git checkout -b master
+    - git checkout -b master && git fetch --unshallow
     # Create/maintain a mirror of all manifest repositories in runner
     # persistent storage, and use that as reference when initializing the
     # build repo, effectively reducing the time spent fetching git repos to