tree_entry path function not working when calling getEntry with a path including subdir

[jfremy]

When building the path of a tree entry, nodegit recursively take the path of each entry's tree parent.
However this does not work if the entry was in a subdir of the parent tree because then we'll be missing the subdir.

As an example:
getEntry("test/rawcommit.js") => entry path will be returned as "rawcommit.js"

I'm submitting a push request + test with a potential fix for discussion