RFR: Add ReadOnlyRepository::files

[Erik Duveblad via github.com]

Hi all,

this patch adds the `ReadOnlyRepository::files` method and also a unit test for the new method.

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

Thanks,
Erik

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Commits:
 - 50ef54f4:	Add ReadOnlyRepository::files

Pull request:
https://git.openjdk.java.net/skara/pull/73

Webrev:
https://webrevs.openjdk.java.net/skara/73/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/73.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/73/head:pull/73