Match the updated merge commit rejection message

The message gained the refname in 77967dc but the test regex still
required a period right after "prohibited".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hsbt

test/test-pre-receive.rb

@@ -97,7 +97,7 @@ def test_prohibit_merge_commit
     make_commit("mame", "mame@ruby-lang.org", "e")
     err = git("push") rescue $!
     assert_match(
-      /A merge commit is prohibited\./,
+      /A merge commit is prohibited for refs\/heads\/master\./,
       err.message
     )
   end