Add JGit as a compile-time dependency
Implement diff-tree using JGit

- Add tests
Throw UnknownRepositoryException for invalid repo

Throw the exception when either the commit id could not be resolved or
the repository is invalid.

- Rename test method names

- Use single quotes for literals and double-quotes for GStrings
Refactor getChanges

- Rename to getModifiedFiles

- Add helper methods: getCommit and getFromCommit

Throw exceptions for the error cases:

- Local repo not found
- Invalid revision string
Use files as input instead of outputStream

- Remove unused outputStream field
Add info logging to GetChangedFiles task
Add logging explaining why the stalk task might be skipped