When checking out with a case-insensitive working directory, we
want to change the case of items in the working directory to
reflect changes that occured in the checkout target.  Diff now
has an option to break case-changing renames into delete/add.

In the prior implementation, enabling GIT_DIFF_UPDATE_INDEX would overwrite
entries in the index with the ones generated from scanning the working if the
OID was the same.

Because this OID comparison ignores file modes, this means an file in the
workdir with only an exec bit difference with the one in the index would end
up being overwritten, resulting in the exec bit being loss. There might be
other related bugs but the fix of comparing OIDs and file modes should
address them all.

The variable noid is guaranteed to be zero at this point of the code path.

This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.

This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.

For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.

The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.

For consistency with the rest of the library, where an opt is an
options *structure*.

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.