The signing callback should not be used; instead, callers should provide
a commit_create_cb, perform the signing and commit creation themselves.

Introduce a new mechanism for `git_rebase_commit` for callers to
customize the experience.  Instead of assuming that we produce the
commit for them, provide a commit creation callback that allows callers
to produce the commit themselves and return the resulting commit id.

Homogenize semantics for atomic-related functions

There were some subtle semantic differences between the various
implementations of atomic functions. Now they behave the same, have
tests and are better documented to avoid this from happening again in
the future.

Of note:

* The semantics chosen for `git_atomic_compare_and_swap` match
  `InterlockedCompareExchangePointer`/`__sync_cal_compare_and_swap` now.
* The semantics chosen for `git_atomic_add` match
  `InterlockedAdd`/`__atomic_add_fetch`.
* `git_atomic_swap` and `git_atomic_load` still have a bit of semantic
  difference with the gcc builtins / msvc interlocked operations, since
  they require an l-value (not a pointer). If desired, this can be
  homogenized.

This reverts commit 487f2a8287c4d1d41d39e05ac53d0e50c679d7e9, reversing
changes made to c6cf7f0e6927f1b264a7dda2467d70b9f744a01f.

GCC C11 warnings

Instead of buf->"typeofbuffer"ReparseBuffer the members will be
referenced with buf->ReparseBuffer."typeofbuffer"

https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer?redirectedfrom=MSDN
calls the union DUMMYUNIONNAME but that looks a bit cluttered.

Introduce GIT_WARN_UNUSED_RESULT