RFR: Use headHash instead of targetRef when fetching PRs

[Erik Helin]

Hi all,

please review this pull request that ensures that the `tester` bot uses `pr.headHash()` instead of `pr.targetRef()` when fetching the changes for a PR (otherwise we just fetch `master` most of the time). I also removed the `targetRef` implementation for `InMemoryPullRequest` and updated the `TestWorkItem` tests to ensure that `TestWorkItem` does _not_ use `pr.targetRef()`.

Thanks,
Erik

## Testing
- [x] `make test` passes on Linux x64

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

Commits:
 - fd5b1e1d: Use headHash instead of targetRef when fetching PRs

Changes: https://git.openjdk.java.net/skara/pull/280/files
 Webrev: https://webrevs.openjdk.java.net/skara/280/webrev.00
  Stats: 10 lines in 3 files changed: 0 ins; 5 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/280.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/280/head:pull/280

PR: https://git.openjdk.java.net/skara/pull/280