Always set `GIT_DIFF_PATCH_DIFFABLE` for all files, regardless of
binary-ness, so that the binary callback is invoked to either
show the binary contents, or just print the standard "Binary files
differ" message.  We may need to do deeper inspection for binary
files where we have avoided loading the contents into a file map.

Introduce a new binary diff callback to provide the actual binary
delta contents to callers.  Create this data from the diff contents
(instead of directly from the ODB) to support binary diffs including
the workdir, not just things coming out of the ODB.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

When we're called with no hunk or line callback, we don't need to
do the hunk or line computation.