[PATCH 0 of 5] Add mecanism to hide changeset

[pierre-yves.david at logilab.fr]

This series offer a minimal implementation to hide some changeset in log and
other relevant command.

The changelog.hide method add revision to the ._hidable set.

All changeset in the ._hidable without descendants outside the set are actually
hidden.

The following stuff are still to be made:

* invalidate content of changelog._hidden when we add a revision
* consider the working directory as non-hidden children

There were also discussion about adding a "reason" argument when hidding
changeset to help debug.

I hope this feature could make is in 1.9 (with and experimental flag) but would
understand the timing is too short.