When no CE branch, find merge-base between CE and HEAD

scripts/ee_specific_check/ee_specific_check.rb

@@ -79,8 +79,8 @@ module EESpecificCheck
   def find_backward_ce_head(ce_fetch_head, ce_fetch_base, ce_merge_base)
     if ce_fetch_head.start_with?('canonical-ce') || # No specific CE branch
         ce_fetch_base == ce_merge_base # Up-to-date, no rebase needed
-      say("CE is up-to-date, using #{ce_fetch_head} directly")
-      ce_merge_base
+      say("CE is up-to-date, using merge-base directly")
+      run_git_command("merge-base #{ce_merge_base} HEAD")
     else
       say("Performing rebase to remove commits in CE haven't merged into EE")
       checkout_and_rebase(ce_merge_base, ce_fetch_base, ce_fetch_head)