This converts blob.c, fileops.c, and all of the win32 files.
Also, various minor cleanups throughout the code.  Plus, in
testing the win32 build, I cleaned up a bunch (although not
all) of the warnings with the 64-bit build.

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>

Ensure that all memory related functions (malloc, calloc, strdup, free,
etc) are using their respective `git__` wrappers.

Our previous assumption that all paths in Windows are encoded in UTF-8
is rather weak, specially when considering that Git is
encoding-agnostic.

These set of functions allow the user to change the library's active
codepage globally, so it is possible to access paths and files on all
international versions of Windows.

Note that the default encoding here is UTF-8 because we assume that 99%
of all Git repositories will be in UTF-8.

Also, if you use non-ascii characters in paths, anywhere, please burn on
a fire.