add new Perl API: Git::Repo, Git::Commit, Git::Tag, and Git::RepoRoot

add new Perl API: Git::Repo, Git::Commit, Git::Tag, and Git::RepoRoot

The Git::Repo class provides low-level access to Git repositories.
The Git::Commit and Git::Tag classes represent commit and tag objects
and allow for accessing their respective properties (like 'author',
'tagger', etc.); Git::Object serves as a common base class for them.
Git::RepoRoot is a helper factory class to instantiate Git::Repo
objects with common properties for repositories under a common root
directory.

Signed-off-by: Lea Wiemann <LeWiemann@gmail.com>