Integrated: vcs: add method Repository.deleteUntrackedFiles

[Erik Helin]

Hi all,

please review this patch that adds a new method to `Repository` - `deleteUntrackedFiles`. As the name suggests, the new
method deletes all untracked files in a repository. This is useful when you want to clean up a repository after for
example a `revert`.

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

Thanks,
Erik

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Commit messages:
 - vcs: add method Repository.deleteUntrackedFiles

Changes: https://git.openjdk.java.net/skara/pull/732/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=732&range=00
  Stats: 48 lines in 4 files changed: 48 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/732.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/732/head:pull/732

PR: https://git.openjdk.java.net/skara/pull/732