tests: don't create new repo inside existing repo in test-rebase-inmemory.t

Differential Revision: https://phab.mercurial-scm.org/D4298

tests/test-rebase-inmemory.t

@@ -156,7 +156,10 Rebase the working copy parent
   |/
   o  0: b173517d0057 'a'
   
+  $ cd ..
+
 Test dry-run rebasing
+
   $ hg init repo3
   $ cd repo3
   $ echo a>a