RFR: git: support unicode filenames

[Erik Helin]

Hi all,

please review this patch that adds support for unicode filenames to `GitRepository`. I also added a unit test
exercising the new functionality.

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

Thanks,
Erik

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

Commit messages:
 - git: support unicode filenames
 - git: commitMetadata should return commits in topo order
 - git: allow diffs against initial revision
 - hg: implement hashes for blobs for ls-tree

Changes: https://git.openjdk.java.net/skara/pull/728/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=728&range=00
  Stats: 45 lines in 4 files changed: 37 ins; 1 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/728.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/728/head:pull/728

PR: https://git.openjdk.java.net/skara/pull/728