RFR: pr: detect clean backports

[Erik Helin]

Hi all,

please review this patch that adds support to `CheckRun` for identifying "clean" backport pull requests. A backport
pull request is "clean" if the diff of the pull request is the same as the diff for the original _modulo_ exact lines
being changed. That is, the same backport pull request must contain the same _changes_ as the original commit, but the
changes don't have to be applied to the exact same lines. The result is equivalent to running `patch` and passing a
fuzzing factor via `--fuzz` that is the same as the number of context lines.

A clean backport also doesn't require reviews. Note though that non-Committers still need a sponsor. I plan to make
this configurable in a follow-up patch.

Testing:
- [x] Added a bunch of tests
- [x] `make test` passes on Linux x64

Thanks,
Erik

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

Commit messages:
 - pr: detect clean backports

Changes: https://git.openjdk.java.net/skara/pull/885/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=885&range=00
  Stats: 634 lines in 5 files changed: 609 ins; 12 del; 13 mod
  Patch: https://git.openjdk.java.net/skara/pull/885.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/885/head:pull/885

PR: https://git.openjdk.java.net/skara/pull/885