Fix head of commit in danger_as_local for drafts and forks

Variable CI_MERGE_REQUEST_SOURCE_BRANCH_SHA is available only in
merged results pipelines. We should fallback to CI_COMMIT_SHA
in other use cases, as merged results pipeline is not used for drafts
and MRs run against forks.

scripts/utils.sh

@@ -172,5 +172,5 @@ function danger_as_local() {
   # Force danger to skip CI source GitLab and fallback to "local only git repo".
   unset GITLAB_CI
   # We need to base SHA to help danger determine the base commit for this shallow clone.
-  bundle exec danger dry_run --fail-on-errors=true --verbose --base="${CI_MERGE_REQUEST_DIFF_BASE_SHA}" --head="${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA}"
+  bundle exec danger dry_run --fail-on-errors=true --verbose --base="${CI_MERGE_REQUEST_DIFF_BASE_SHA}" --head="${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA:-$CI_COMMIT_SHA}"
 }