RFR: forge: add method PullRequest.diff

[Erik Helin]

Hi all,

please review this patch that adds the method `PullRequest.diff` for fetching the diff between a pull request's `HEAD`
and target branch.

Thanks,
Erik

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Commit messages:
 - forge: add method PullRequest.diff

Changes: https://git.openjdk.java.net/skara/pull/882/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=882&range=00
  Stats: 51 lines in 6 files changed: 46 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/882.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/882/head:pull/882

PR: https://git.openjdk.java.net/skara/pull/882