RFR: forge: add method diffUrl to PullRequest

[Erik Helin]

Hi all,

please review this small patch that adds the method `PullRequest::diffUrl`.
Although the implementation is the same for both `GitHubPullRequest` and
`GitLabMergeRequest` I chose to not do a `default` method to highlight that this
is more of a coincidence than a rule.

Thanks,
Erik

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Commits:
 - 89faf8e2: forge: add method diffUrl to PullRequest

Changes: https://git.openjdk.java.net/skara/pull/459/files
 Webrev: https://webrevs.openjdk.java.net/skara/459/webrev.00
  Stats: 32 lines in 6 files changed: 25 ins; 4 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/459.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/459/head:pull/459

PR: https://git.openjdk.java.net/skara/pull/459