RFR: vcs: add DiffComparator for comparing diffs

[Erik Helin]

Hi all,

please review this patch that adds the new class `DiffComparator` in `vcs` for comparing `Diff`s. I've also updated `CheckRun` to make use of this new class.

Thanks,
Erik

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

Commit messages:
 - vcs: add DiffComparator for comparing diffs

Changes: https://git.openjdk.java.net/skara/pull/951/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=951&range=00
  Stats: 189 lines in 2 files changed: 105 ins; 82 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/951.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/951/head:pull/951

PR: https://git.openjdk.java.net/skara/pull/951