RFR: Add ReadOnlyRepository::contains

[Erik Duveblad via github.com]

Hi all,

this patch adds `ReadOnlyRepository::contains` that checks if a given hash `h` resides in the given branch `b`. I also added small unit test case, it was hard to add more elaborate tests given that `HgRepository::branch` creates a bookmark and not a branch.

## Testing
- [x] `sh gradlew test` passes
- [x] Added a small unit test

Thanks,
Erik

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Commits:
 - d0559535:	Add ReadOnlyRepository::contains

Pull request:
https://git.openjdk.java.net/skara/pull/96

Webrev:
https://webrevs.openjdk.java.net/skara/96/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/96.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/96/head:pull/96