RFR: hg: implement ReadOnlyRepository.isAncestor

[Erik Helin]

Hi all,

please review this patch that implements `ReadOnlyRepository.isAncestor` for `HgRepository`.

Testing:
- [x] `make test` passes on Linux x64
- [x] Added a new unit test

Thanks,
Erik

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Commit messages:
 - hg: implement ReadOnlyRepository.isAncestor

Changes: https://git.openjdk.java.net/skara/pull/730/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=730&range=00
  Stats: 29 lines in 2 files changed: 28 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/730.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/730/head:pull/730

PR: https://git.openjdk.java.net/skara/pull/730