RFR: 438: Add "git pr issue" subcommand

[Erik Helin]

Hi all,

please review this patch that adds the `git pr issue` subcommand (corresponding
to the `/issue` pull request command). A user can now run e.g. `git pr issue
--add=SKARA-419` instead of the pull request command `/issue add SKARA-419`.

Testing:
- [x] Manual testing of Linux x64

Thanks,
Erik

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Commit messages:
 - Add git pr issue

Changes: https://git.openjdk.java.net/skara/pull/684/files
 Webrev: https://webrevs.openjdk.java.net/skara/684/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-438
  Stats: 151 lines in 3 files changed: 150 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/684.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/684/head:pull/684

PR: https://git.openjdk.java.net/skara/pull/684