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.

remove /ZM1000 from msvc builds

This adds a `git_diff_patch_print()` API which is more like the
existing API to "print" a patch from an entire `git_diff_list`
but operates on a single `git_diff_patch` object.

Also, it rewrites the `git_diff_patch_to_str()` API to use that
function (making it very small).

Updates config upon moving and deletion of branches

Pedantic ordering of GIT_UNUSED vs. variable declarations.