Arguably all uses of readdir_r are unnecessary, but in this case
especially so, as the directory handle only exists within this function,
so we don't race with anybody.

Using FindFirstFile and FindNextFile in win32 allows us to
use the directory information that is returned, instead of
us having to get the file attributes all over again, which
is a distinct cost savings on win32.

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.

Introduce a new `git_path_diriter` that can iterate directories
efficiently for each platform.

The documentation for `git_path_join_unrooted` states that the base
length will be returned, so that consumers like checkout know where
to start creating directories instead of always creating directories
at the directory root.

Validate HFS ignored char ".git" paths when `core.protectHFS` is
specified.  Validate NTFS invalid ".git" paths when `core.protectNTFS`
is specified.

HFS filesystems ignore some characters like U+200C.  When these
characters are included in a path, they will be ignored for the
purposes of comparison with other paths.  Thus, if you have a ".git"
folder, a folder of ".git<U+200C>" will also match.  Protect our
".git" folder by ensuring that ".git<U+200C>" and friends do not match it.