gitlab-ci: If there is not old sha check last 20 commits

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

.gitlab-ci.yml

@@ -90,7 +90,7 @@ fedora/csbuild:
   script:
   - |
     if [[ -z "$CI_COMMIT_BEFORE_SHA" ]]; then
-        export CI_COMMIT_BEFORE_SHA=$(git rev-parse HEAD^)
+        export CI_COMMIT_BEFORE_SHA=$(git rev-parse HEAD~20)
     fi
 
     # Check if the commit exists in this branch