Revision numbers/raw_id/id

[lukaszb]

We need to clean up changeset id representation. All backends should support integers as revision numbers.

As docstrings state, revision should always be integers. On the other hand, "raw_id" should be just full hash (or simply integer for subversion, for instance). This would probably need some code refactor (but "grin raw_id" doesn't output much...).

This would allow one to easily get numbers for git commits, just like calling:
{{{
git log --pretty=oneline | awk '{ print NR-1 ":" $1 }' | sort -rn | awk -F":" '{ print NR-1 ":" $2 }' | sort -rn
}}}
in any git repo.

---

Reference: https://bitbucket.org/marcinkuzminski/vcs/issues/25/