RFR: Add Repository::revert

[Erik Duveblad via github.com]

Hi all,

this small patch adds the `Repository::revert` method, the corresponding git and hg implementations and a small test.

## Testing
- [x] `sh gradlew test` passes on Linux x86_64

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Commits:
 - 7e30d6c3:	Add Repository::revert

Pull request:
https://git.openjdk.java.net/skara/pull/60

Webrev:
https://webrevs.openjdk.java.net/skara/60/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/60.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/60/head:pull/60