RFR: 54: Add a mirror bot

[Erik Duveblad via github.com]

Hi all,

this patch adds a bot that can mirror two repositories, including multiple branches and multiple tags (in contrast to the forwarder bot that only forwards between two branches).

# Testing
- [x] `sh gradlew test` passes
- [x] Added four new unit tests to test the mirror bot

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Commits:
 - 9f1b7a26:	54: Add a mirror bot

Pull request:
https://git.openjdk.java.net/skara/pull/41

Webrev:
https://webrevs.openjdk.java.net/skara/41/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/41.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/41/head:pull/41