RFR: pr: add /backport commit command

[Erik Helin]

Hi all,

please review this patch that adds the `/backport` [commit command](https://wiki.openjdk.java.net/display/SKARA/Commit+Commands#CommitCommands-/backport).

Testing:
- [x] Added four new unit tests
- [x] `make test` passes on Linux x64

Thanks,
Erik

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

Commit messages:
 - pr: add /backport command

Changes: https://git.openjdk.java.net/skara/pull/945/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=945&range=00
  Stats: 468 lines in 7 files changed: 464 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/945.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/945/head:pull/945

PR: https://git.openjdk.java.net/skara/pull/945