RFR: vcs: add Hash::zero

[Erik Helin]

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

-------------

Commits:
 - 2c2c69c7: vcs: add Hash::zero

Changes: https://git.openjdk.java.net/skara/pull/469/files
 Webrev: https://webrevs.openjdk.java.net/skara/469/webrev.00
  Stats: 50 lines in 13 files changed: 5 ins; 9 del; 36 mod
  Patch: https://git.openjdk.java.net/skara/pull/469.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/469/head:pull/469

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