Use the current merge base between CE MR and EE MR

In most cases, they're the same as the merge base
between CE master and EE master. However it could be
different. For example we might be merging something
into EE MR, which isn't landed on EE master yet.

This will improve that case.

scripts/ee_specific_check/ee_specific_check.rb

@@ -33,8 +33,8 @@ module EESpecificCheck
     ce_fetch_head = fetch_remote_ce_branch
     ee_fetch_head = head_commit_sha
     ce_fetch_base = run_git_command("merge-base canonical-ce/master #{ce_fetch_head}")
-    ce_merge_base = run_git_command("merge-base canonical-ce/master canonical-ee/master")
     ee_fetch_base = run_git_command("merge-base canonical-ee/master HEAD")
+    ce_merge_base = run_git_command("merge-base #{ce_fetch_head} #{ee_fetch_head}")
 
     ce_updated_head =
       find_ce_compare_head(ce_fetch_head, ce_fetch_base, ce_merge_base)