Ensures that commits from parent(*) has a repository by rcjsuen · Pull Request #1658 · nodegit/nodegit

Commit has functions that requires a reference to a repository to run.
Because parent(*) was simply calling out to libgit2's
git_commit_parent directly, its repo field was not being set. Creating
a wrapper in the Commit class and assigning a repository to the object
before returning it will fix this problem.

Signed-off-by: Remy Suen <remy.suen@gmail.com>