RFR: Use ZonedDateTime for date in Commit

[Erik Duveblad via github.com]

Hi,

this patch changes from `Instant` to `ZonedDateTime` in `CommitMetadata`. This is done to preserve the time zone recorded in the underlying VCS commit (both git and hg store the time zone of the developer's machine in the commit).

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

Thanks,
Erik

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Commits:
 - 1d5b3cc7:	Use ZonedDateTime for date in Commit

Pull request:
https://git.openjdk.java.net/skara/pull/48

Webrev:
https://webrevs.openjdk.java.net/skara/48/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/48.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/48/head:pull/48