RFR: pr: add /tag commit command

[Erik Helin]

Hi all,

please review this patch that adds the `/tag` commit command. As the name implies, the command is used to create a tag with the given name referring to the commit the command is applied to. The command will check that the tag isn't already used and that it conforms to an eventual `.jcheck/conf` configuration.

Thanks,
Erik

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Commit messages:
 - pr: add /tag commit command

Changes: https://git.openjdk.java.net/skara/pull/1056/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1056&range=00
  Stats: 426 lines in 3 files changed: 425 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1056.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1056/head:pull/1056

PR: https://git.openjdk.java.net/skara/pull/1056