RFR: Lazily request repository JSON for GitHub

[Erik Helin]

Hi all,

please review this small pull request that ensures that we lazily request project info for an instance of `GitHubRepository`. It is quite common to only use a `GitHubRepository` instance to request for example all pull requests, a call which does need the project info to be populated. This patch saves us a network request in such scenarios. The caching is safe since an instance `GitHubRepository` can only be accessed by a single thread at a time.

Thanks,
Erik

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Commits:
 - f451862c: Lazily request repository JSON for GitHub

Changes: https://git.openjdk.java.net/skara/pull/296/files
 Webrev: https://webrevs.openjdk.java.net/skara/296/webrev.00
  Stats: 14 lines in 1 file changed: 9 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/296.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/296/head:pull/296

PR: https://git.openjdk.java.net/skara/pull/296