RFR: git-pr: move subcommands to separate files

[Erik Helin]

Hi all,

please review this patch that splits `GitPr.java` into multiple files - one file
per sub-command. This makes the code much more maintainable and allows for easy
addition of new commands.

Testing:
- Manual testing of `git pr`

Thanks,
Erik

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

Commits:
 - 365b1ce4: git-pr: move subcommands to separate files

Changes: https://git.openjdk.java.net/skara/pull/460/files
 Webrev: https://webrevs.openjdk.java.net/skara/460/webrev.00
  Stats: 3621 lines in 18 files changed: 2023 ins; 1569 del; 29 mod
  Patch: https://git.openjdk.java.net/skara/pull/460.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/460/head:pull/460

PR: https://git.openjdk.java.net/skara/pull/460