This is just a bunch of small fixes that I noticed while looking
at the UTF8 and UTF16 path stuff.  It fixes a slowdown in looking
for an empty directory (not exiting loop asap), makes the dir name
in the git__DIR structure be a GIT_FLEX_ARRAY to save an allocation,
and fixes some slightly odd assumptions in the cl_getenv helper.

Also fixed up call-sites to use the correct buffer sizes, especially
when converting to utf-8.

This is a major reorganization of the diff code.  This changes
the diff functions to use the iterators for traversing the
content.  This allowed a lot of code to be simplified.  Also,
this moved the functions relating to outputting a diff into a
new file (diff_output.c).

This includes a number of other changes - adding utility
functions, extending iterators, etc. plus more tests for the
diff code.  This also takes the example diff.c program much
further in terms of emulating git-diff command line options.

This fixes the missing readdir_r from win32 and fixes other
platforms to always use the reentrant readdir_r form for reading
directory contents.

Signed-off-by: schu <schu-github@schulog.org>

This takes all of the functions that look up simple data about
paths (such as `git_futils_isdir`) and moves them over to path.h
(becoming `git_path_isdir`).  This leaves fileops.h just with
functions that actually manipulate the filesystem or look at
the file contents in some way.

As part of this, the dir.h header which is really just for win32
support was moved into win32 (with some minor changes).