Deprecate Repository.revision attribute - we should use id specific for each backends

[lukaszb]

Currently Repository.get_changeset accepts revision number (given as integer) which is generally wrong for both hg and git.

Mercurial can operate rather easily with that however for instance one may push repository to the server on which vcs process repositories management, then tries to use i.e. 10 as changeset id and gets different results from local repository. With integers there are integrity problems and only full hexes identifies changesets properly.

Git doesn't support integers as id numbers for commits in any way (there are some possibility to write own script but it would works properly only for new repositories which is not the case mainly).

That said, we should deprecate integer identification. This is great feature but it just is not proper one for repository management/browsing.

Applications should implement such feature on its own.

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Reference: https://bitbucket.org/marcinkuzminski/vcs/issues/43/