Uses a distributed lock on git handler to avoid race condition about waliki

[mgaitan]

this seems to be simple enough https://pypi.python.org/pypi/pylock/0.3

[wagnerflo]

From my reading of pylock's documentation this package seems a bad choice. Anyone deploying a Waliki based website will then have to decide between:

1. Increased effort for installing and maintaining either Redis or Memcache.
2. Selecting the "Open (non-locking) backend" and gaining no improvement while still inheriting the increased complexity in Waliki code as well as more dependencies.

A distributed locking library that has a "fallback" to file locks for simple and small installations would be very much preferable IMO.