RFR: Add submodule and subrepo support

[Erik Helin]

Hi all,

this patch adds initial submodule and subrepo support to `GitRepository` and `HgRepository` and also adds a few unit tests.

Thanks,
Erik

## Testing
- `sh gradlew test` passes on Linux x64

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Commits:
 - 3506616c: Add submodule and subrepo support

Changes: https://git.openjdk.java.net/skara/pull/217/files
 Webrev: https://webrevs.openjdk.java.net/skara/217/webrev.00
  Stats: 212 lines in 7 files changed: 212 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/217.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/217/head:pull/217

PR: https://git.openjdk.java.net/skara/pull/217