Just pick the CE merge base when there's no CE branch

I put `git_run_command("merge-base #{ce_merge_base} HEAD")`
before in case the EE branch was behind the CE merge-base.
When that happened, I think something would be wrong.
However, I just tried to reproduce the issue I hit before,
and it looks like I can't really reproduce it now,
and just using the CE merge base is working properly.

Well, I think even this is not completely solving the issue,
it's at least better than the current status.

Let's see...

scripts/ee_specific_check/ee_specific_check.rb

@@ -79,7 +79,7 @@ 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
       say("No CE branch found, using merge base directly")
-      run_git_command("merge-base #{ce_merge_base} HEAD")
+      ce_merge_base
     elsif ce_fetch_base == ce_merge_base # Up-to-date, no rebase needed
       say("EE is up-to-date with CE, using #{ce_fetch_head} directly")
       ce_fetch_head