Improve docs, examples, warnings

Some more changes to the Index API

This improves docs in some of the public header files, cleans
up and improves some of the example code, and fixes a couple
of pedantic warnings in places.

Why Apple, why?

Update of text stats calculation

Do not interpret 0x85 as Next Line (NEL)
char when gathering statistics for a text file.

Reorganize transport architecture

This adds a new API that allows users to reload the config if the
file has changed on disk.  A new config callback function to
refresh the config was added.

The modified time and file size are used to test if the file needs
to be reloaded (and are now stored in the disk backend object).

In writing tests, just using mtime was a problem / race, so I
wanted to check file size as well.  To support that, I extended
`git_futils_readbuffer_updated` to optionally check file size in
addition to mtime, and I added a new function `git_filebuf_stats`
to fetch the mtime and size for an open filebuf (so that the
config could be easily refreshed after a write).

Lastly, I moved some similar file checking code for attributes
into filebuf.  It is still only being used for attrs, but it
seems potentially reusable, so I thought I'd move it over.

Add git_diff_patch_to_str API

repo: enhance git_repository_state() detection

Index changes

This memset was being reached after the entire packfile under
WinHttp, so the byte count was being lost for small repos.

Cmake flags clean

Stash

- No overwriting allows control from cmake cache or cmdline
- -g is already the CMake default
- -O0 is already gcc's default

- Those are the RelWithDebInfo flags.
- They should be controlled from CMAKE_BUILD_TYPE

- For Debug builds, CMake uses concatenated
  CMAKE_C_FLAGS and CMAKE_C_FLAGS_DEBUG
- This reverts commit 291f7122.