RFR: vcs: add Hash::zero

[Robin Westberg]

On Mon, 24 Feb 2020 12:56:33 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this small refactoring that adds the method `Hash::zero`
> representing an empty hash. The zero hash was already used throughout the code
> base but in a less clear way.
> 
> Testing:
> - `make test` passes on Linux x64
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/469