diriter: test we can iterate root
Ensure that we can iterate the filesystem root and that paths come
back well-formed, not with an additional '/'. (eg, when iterating
`c:/`, expect that we do not get some path like `c://autoexec.bat`).
git_path_diriter: next shouldn't take path ptr
The _next method shouldn't take a path pointer (and a path_len
pointer) as 100% of current users use the full path and ignore
the filename.
Plus let's add some docs and a unit test.