Log Message:
Upgrade to 2.1.1, from Jan Danielsson in private email.

Work has gone in to reducing memory usage in the 2.1 series.

This is a small bugfix release. Upgrading is recommended for anyone
running 2.1.0 or earlier.

Bugfixes:
* Avoid malloc(0) in patiencediff, which is non-portable.
* Fix stub sftp test server to call os.getcwdu().
* Merge correctly when this_tree is not a WorkingTree.
* Warn if pyrex is too old to compile the new SimpleSet and StaticTuple
  extensions, rather than having the build fail randomly.