RFR: git: allow diffs against initial revision

[Erik Helin]

Hi all,

please review this patch that makes it possible to do `diff` and `status` against the initial revision of a git
repository.

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

Thanks,
Erik

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Commit messages:
 - git: allow diffs against initial revision

Changes: https://git.openjdk.java.net/skara/pull/726/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=726&range=00
  Stats: 56 lines in 2 files changed: 53 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/726.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/726/head:pull/726

PR: https://git.openjdk.java.net/skara/pull/726