Enable use and display of git revision numbers

A thing I've noticed while using hg-git is that when I interact with git users, maybe I'll be talking to another developer or I'll be looking at the github page, periodically someone will say something about "revision 4898e" and I'll have no idea what that means and no way to look it up, because all the hashes in MY copy of the repository are in hg format. The git users and I do not have a common way of referring to revisions that we both understand, unless someone wrote really good commit messages. In an attempt to address this I made three changes:

* There is now an `hg gsummary` or `hg gsum` which prints the git-equivalent revision number (if any) of the current working directory state. It looks like:

```git-rev: 4898e77458a287789dd5f42b51685f84d1fdf431```

* You may now, in any hg command which would normally take an hg revision number, hg revision hash, or tag, just haul off and use a git revision number, like `hg up 4898e` to get the git revision from my example above. 

(A small note: If you say `hg up 4898e`, and 4898e as a prefix just happens to match both an hg revision and a git revision, the current code will silently use the hg revision. You can specify you wanted the git revision by using a "g" prefix to the revision number, like `hg up g4898e`.)

* Most commands which show information about a changeset will now voluntarily mention the git revision number (if any) along with the tags, etc for that changeset. For example `hg sum` now looks like:

```parent: 432:2740d263255c git:4898e77458a2 default/default/master default/master tip master
 Mercurial 1.8 compatibility for git-rev show
branch: default
commit: 1832 unknown (clean)
update: (current)```

And lines in `hg log` now look like:

```changeset:   431:9d486e95cf37
git-rev:     bd372664271b
user:        mcc <andrew.mcclure@gmail.com>
date:        Fri Sep 02 00:49:22 2011 -0700
files:       hggit/__init__.py hggit/hgrepo.py
description:
git-rev code cleanup```

The overall goal was to make addressing a revision by git-revision number as seamless as addressing it by tag or bookmark.

The patch also adds to the tests/ directory a small script I wrote named virtualenv-setup.py which uses virtualenv to locally deploy some self-contained copies of mercurial using specific known versions of mercurial and dulwich. This might be redundant with some of the existing test scripts but I found it helpful. I used this to test this checkin back to 1.8.

Does this sound good? Two caveats I have noticed:

- The hg sum / hg log additions only appear if you are not using styles or templates. I think this is desirable, BUT I think that some change should be made such that *if* someone wants to write a template which is aware of hg-git, there should be some way for that template to read the git-revision of a changeset it is displaying. I don't know a lot about the template system though so I couldn't really figure out how to do this (though I've made a post to the mercurial mailing list asking for help).

- For reasons I don't really understand, this doesn't work with `hg outgoing`. git revisions are not displayed in hg outgoing even if they are known.

EDIT: Here's my mercurial mailing list post btw, it includes gory implementation details for the above if that interests you. http://selenic.com/pipermail/mercurial-devel/2011-September/034064.html