evolve: unify revision handling and rework error message
This changesets do a massive refactoring of the way evolve select changesets to
be evolve. We now use a two stages approach:
1) read command line argument and produce a list of revision to evolve.
2) evolve all these revisions.
This allow a much cleaner, robust and extensible code. In the process the error
message issued when there is nothing to evolve have been updated to informs
about other troubles in the repository and point at useful option to solve them.
The 'update' case is handled independently at the start of the function.