RFR: 205: git-pr: allow configurable default action

[Erik Helin]

Hi all,

please review this small patch that makes the default action of `git pr`
configurable.

Thanks,
Erik

## Testing
- [x] Manual testing of `git pr` with and without a default action

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Commits:
 - 4821a25d: skara-205

Changes: https://git.openjdk.java.net/skara/pull/332/files
 Webrev: https://webrevs.openjdk.java.net/skara/332/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-205
  Stats: 9 lines in 1 file changed: 7 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/332.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/332/head:pull/332

PR: https://git.openjdk.java.net/skara/pull/332