RFR: git-pr: add the sponsor subcommand

[Erik Helin]

Hi all,

please review this patch that adds the `git pr sponsor` subcommand. The command
`git pr sponsor` can be used for integrating pull requests that needs
sponsoring.

Thanks,
Erik

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Commit messages:
 - git-pr: add the sponsor subcommand

Changes: https://git.openjdk.java.net/skara/pull/485/files
 Webrev: https://webrevs.openjdk.java.net/skara/485/webrev.00
  Stats: 107 lines in 2 files changed: 107 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/485.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/485/head:pull/485

PR: https://git.openjdk.java.net/skara/pull/485