gitlab-ci: Fix csbuild to run latest 20 commits

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

.gitlab-ci.yml

@@ -90,12 +90,12 @@ fedora/csbuild:
   script:
   - |
     if [[ -z "$CI_COMMIT_BEFORE_SHA" ]]; then
-        export CI_COMMIT_BEFORE_SHA=$(git rev-parse HEAD~20)
+        export CI_COMMIT_BEFORE_SHA=$(git rev-parse "${CI_COMMIT_SHA}~20")
     fi
 
     # Check if the commit exists in this branch
     # This is not the case for a force push
-    git branch --contains $CI_COMMIT_BEFORE_SHA 2>/dev/null || export CI_COMMIT_BEFORE_SHA=$(git rev-parse HEAD^)
+    git branch --contains $CI_COMMIT_BEFORE_SHA 2>/dev/null || export CI_COMMIT_BEFORE_SHA=$(git rev-parse "${CI_COMMIT_SHA}~20")
 
     export CI_COMMIT_RANGE="$CI_COMMIT_BEFORE_SHA..$CI_COMMIT_SHA"