RFR: git-pr: friendly help message when missing action

[Erik Helin]

Hi all,

please review this small patch that makes `git pr` print a more friendly help
message when no action is specified.

Testing:
- Manual testing of `git pr`

Thanks,
Erik

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

Commits:
 - 29e950c6: git-pr: friendly help message when missing action

Changes: https://git.openjdk.java.net/skara/pull/405/files
 Webrev: https://webrevs.openjdk.java.net/skara/405/webrev.00
  Stats: 12 lines in 1 file changed: 11 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/405.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/405/head:pull/405

PR: https://git.openjdk.java.net/skara/pull/405