RFR: Enable Mercurial support for "pr create"

[Erik Helin]

Hi all,

this patch add Mercurial (hg) support for the `pr create` command (this PR was
craeted by `hg pr create`). I know the code is a bit messy at the moment, I will
clean it up in a later PR (I wanted to get all pieces into place before refactoring).

Testing:
- [x] `sh gradlew test` passes (otoh there are no tests GitPr.java)
- [x] Manual testing with `hg pr create`

Thanks,
Erik

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

Commits:
 - ae53679c:	Enable Mercurial support for "pr create"

Pull request:
https://git.openjdk.java.net/skara/pull/129

Webrev:
https://webrevs.openjdk.java.net/skara/129/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/129.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/129/head:pull/129