changegroup: flush the ui stdio buffers after adding a changegroup

This eliminates the following test failure on Windows, as well as a similar one
in evolve's test-wireproto.t.  See the previous patch for details on the
problem.

  --- e:/Projects/hg/tests/test-init.t
  +++ e:/Projects/hg/tests/test-init.t.err
  @@ -216,10 +216,10 @@
      * test                      0:08b9e9f63b32
     $ hg clone -e "python \"$TESTDIR/dummyssh\"" local ssh://user@dummy/remote-bookmarks
     searching for changes
  +  exporting bookmark test
     remote: adding changesets
     remote: adding manifests
     remote: adding file changes
     remote: added 1 changesets with 1 changes to 1 files
  -  exporting bookmark test
     $ hg -R remote-bookmarks bookmarks
        test                      0:08b9e9f63b32

mercurial/changegroup.py

@@ -891,6 +891,7 b' def addchangegroup(repo, source, srctype'
 
     finally:
         tr.release()
+        repo.ui.flush()
     # never return 0 here:
     if dh < 0:
         return dh - 1