tests: demonstrate a bug blocking a redundant branch close

tests/test-branches.t

@@ -292,6 +292,26 b' verify update will accept invalid legacy'
   [10]
   $ hg commit -d '9 0' --close-branch -m 're-closing this branch' b
 
+  $ echo baz > b
+  $ hg log -r .
+  changeset:   14:acc63dd7207f
+  branch:      b
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:09 1970 +0000
+  summary:     re-closing this branch
+  
+  $ hg commit -d '9 0' --close-branch -m 'empty re-closing this branch' -X b
+  $ hg log -r .
+  changeset:   15:3c41ebd8ea8b
+  branch:      b
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:09 1970 +0000
+  summary:     empty re-closing this branch
+  
+  $ hg revert b
+
   $ hg debugstrip --rev 13: --no-backup
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg revert --all --no-backup