gitlab: restrict git history to 100 commits

We don't need the full git history when running CI jobs. From a code POV
we only need the most recent commit, but we want to be able to run
checks on the commits too. In particular to validate the DCO signoff for
each commit.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

 variables:
   MAKE: make
+  GIT_DEPTH: 100
 
 .job_template: &job_definition
   script: