RFR: pr: add /backport commit command [v2]

[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

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

  Wording

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

Changes:
  - all: https://git.openjdk.java.net/skara/pull/945/files
  - new: https://git.openjdk.java.net/skara/pull/945/files/e9860704..deefe5fe

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=945&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=945&range=00-01

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