[PATCH 0 of 3] Improve pushing of named branches (issue736)

[Sune Foldager]

This is a re-issue of the original push-namedbranch patch.
It has been split into three separate patches as follows:

1. The branchmap repository and wire command.
2. The improve pre-push logic and test updates.
3. Tests for the new pre-push logic.

The patches are also available here:
https://bitbucket.org/cyanite/hg-crew-mq/