RFR: Add support for Mercurial

[Erik Duveblad via github.com]

Hi all,

this patch adds skara.py which makes it quite a bit more convenient to use the Skara tools from Mercurial (hg). I also added `--mercurial` to `git-pr` and `git-fork` so they can be used with [hg-git](https://bitbucket.org/durin42/hg-git). There are still a few minor things to work out, for example `hg pr create` does not work yet and I need to do add some help texts, but it is a start.

## Testing
- [x] Manual testing with git token

Thanks,
Erik

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Commits:
 - fadd0cd8:	Add support for Mercurial

Pull request:
https://git.openjdk.java.net/skara/pull/122

Webrev:
https://webrevs.openjdk.java.net/skara/122/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/122.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/122/head:pull/122