[skara] RFR: forge: add HostedRepository.createPullRequestUrl

[Erik Helin]

Hi all,

please review this small patch that adds the `HostedRepository.createPullRequestUrl` for getting the URL that a user can click to create a pull request between two `HostedRepository` instances.

Thanks,
Erik

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Depends on: https://git.openjdk.java.net/skara/pull/1027

Commit messages:
 - forge: add HostedRepository.createPullRequestUrl

Changes: https://git.openjdk.java.net/skara/pull/1028/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1028&range=00
  Stats: 32 lines in 5 files changed: 32 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1028.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1028/head:pull/1028

PR: https://git.openjdk.java.net/skara/pull/1028