- Implements some of the functionality provided by `git ls-tree`.
- Given the additional parsing of arguments, copying what libgit2 did
  and use `structopt` as an additional dev-dependencies.

- A rather naive first cut.

- Also, output updated to be equivalent to the one produced by running
  `git log --full-history <commit> <path>`.
- Shouldn't have mixed author/commit time (it was always author time
  being displayed by default).

- Unless reverse is required - the git log DAG is unidirectional, the
  whole graph must be read in order for it to be reversed - libgit2 does
  this internally when the `GIT_SORT_REVERSE` is specified.

  See: https://github.com/libgit2/libgit2/blob/v1.6.4/src/libgit2/revwalk.c#L659-L669
- Include some comments that might hopefully help future readers.

- This set of filters will report (non)merge commits by filtering out
  commits with min/max number of parents
- Also refactored the path filter logic and restructure the layout of
  the code block plus add in comments to make fit as an example.

- Also remember to test for empty list of paths and permit that through
  the filter as `any()` returns false on empty - likewise this applies
  to the check on list of parent_ids.

…plications.

Please note that these are just examples, which aren't necessarily
production ready in terms of quality or performance.