RFR: Implement HgRepository::amend

[Erik Duveblad via github.com]

Hi all,

this patch implements the `HgRepository::amend` method. I also added a test that tests `amend` for both git and hg.

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

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Commits:
 - 71734d51:	Implement HgRepository::amend

Pull request:
https://git.openjdk.java.net/skara/pull/59

Webrev:
https://webrevs.openjdk.java.net/skara/59/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/59.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/59/head:pull/59