RFR: pr: show diff of diffs for backports

[Erik Helin]

Hi all,

please review this patch that shows a "diff of diffs" for backports - i.e. the backport commit will be compared to the original commit and the differences will be shown (for non-clean backports). I also added some unit tests for the new `DiffComparator` utility.

Thanks,
Erik

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Commit messages:
 - pr: show diff of diffs for backports

Changes: https://git.openjdk.java.net/skara/pull/973/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=973&range=00
  Stats: 389 lines in 6 files changed: 373 ins; 0 del; 16 mod
  Patch: https://git.openjdk.java.net/skara/pull/973.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/973/head:pull/973

PR: https://git.openjdk.java.net/skara/pull/973