RFR: hg: use --template instead of ext.py for metadata

[Erik Helin]

Hi all,

please review this patch that makes `HgRepository.commitMetadata` use `--template` instead of `ext.py` for generating
the metadata.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - hg: use --template instead of ext.py for metadata

Changes: https://git.openjdk.java.net/skara/pull/739/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=739&range=00
  Stats: 43 lines in 2 files changed: 33 ins; 3 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/739.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/739/head:pull/739

PR: https://git.openjdk.java.net/skara/pull/739