RFR: 209: git-pr: add ISSUE column to git-pr list

[Erik Helin]

Hi all,

please review this patch that adds an "ISSUES" column to `git pr list` showing
the JBS issue ids a pull request are associated with. A future cleanup should
probably extract the pull request body parsing logic into a separate package so
it can be reused throughout Skara.

Thanks,
Erik

## Testing
- [x] Manual testing of `git pr list`

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

Commits:
 - 023d9b9a: skara-209

Changes: https://git.openjdk.java.net/skara/pull/337/files
 Webrev: https://webrevs.openjdk.java.net/skara/337/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-209
  Stats: 53 lines in 1 file changed: 49 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/337.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/337/head:pull/337

PR: https://git.openjdk.java.net/skara/pull/337