RFR: git-pr: git pr sponsor should sponsor

[Erik Helin]

Hi all,

please review this patch that makes `git pr sponsor` actually run `GitPrSponsor::main`, _not_ `GitPrSet::main`.

Thanks,
Erik

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Commit messages:
 - git-pr: git pr sponsor should sponsor

Changes: https://git.openjdk.java.net/skara/pull/791/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=791&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/791.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/791/head:pull/791

PR: https://git.openjdk.java.net/skara/pull/791