RFR: pr: detect clean backports [v2]

[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

Erik Helin has updated the pull request incrementally with one additional commit since the last revision:

  Fix spelling
  
  Co-authored-by: Robin Westberg <robin at westberg.se>

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

Changes:
  - all: https://git.openjdk.java.net/skara/pull/885/files
  - new: https://git.openjdk.java.net/skara/pull/885/files/c33ec6fe..49542d66

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=885&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=885&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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