RFR: Better Mercurial compatability

[Erik Duveblad via github.com]

Hi all,

this patch improves the Mercurial (hg) support for `git fork` and `git pr`. Now all subcommands work with `hg pr` _except_ for `hg pr create` which I haven't fixed yet.

## Testing
- [x] `sh gradlew test` passes on Linux x86_64
- [x] Manual testing of `hg pr`, `hg fork`, `hg webrev` and `hg jcheck`

Thanks,
Erik

----------------

Commits:
 - 790a7d05:	Better Mercurial compatability

Pull request:
https://git.openjdk.java.net/skara/pull/125

Webrev:
https://webrevs.openjdk.java.net/skara/125/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/125.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/125/head:pull/125