RFR: git-pr: update git-pr -h

[Erik Helin]

Hi all,

please review this small update to `git pr -h` that adds some missing sub-commands to the help output.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - git-pr: update git-pr -h

Changes: https://git.openjdk.java.net/skara/pull/953/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=953&range=00
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/953.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/953/head:pull/953

PR: https://git.openjdk.java.net/skara/pull/953