autocommit

[Georges Racinet]

With the recent introduction of merge-ins, it becomes especially interesting to have automatic commit (of the merges) and pushes.

The recipe could optionally produce two scripts for that purpose ``autocommit_PART`` and ``autopush_PART``
These could in turn be put in a CI robot, to be run after the tests are completed.

In that perspective, ``autopush`` should rollback the commits that could not be pushed, so that next run's vcs-clean will start afresh.

I'll work on this in a dedicated branch.

[Georges Racinet (gracinet)]

First row of merges has been pushed by the script produced by the implementation branch.
Will use it manually for a few days then have buildbots execute it, then merge (in about a week).