RFR: Initial implementation of commit commands

[Robin Westberg]

This is the initial implementation of support for commands issued directly on commits (and integrated pull requests).

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Commit messages:
 - Fix trailing whitespace
 - Missing copyright headers
 - Initial implementation of commit commands

Changes: https://git.openjdk.java.net/skara/pull/913/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=913&range=00
  Stats: 624 lines in 10 files changed: 528 ins; 77 del; 19 mod
  Patch: https://git.openjdk.java.net/skara/pull/913.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/913/head:pull/913

PR: https://git.openjdk.java.net/skara/pull/913