combine-diff: drop public declaration of combine_diff_path_size()

combine-diff: drop public declaration of combine_diff_path_size()

We want callers to use combine_diff_path_new() to allocate structs,
rather than using combine_diff_path_size() and xmalloc(). That gives us
more consistency over the initialization of the fields.

Now that the final external user of combine_diff_path_size() is gone, we
can stop declaring it publicly. And since our constructor is the only
caller, we can just inline it there.

Breaking the size computation into two parts also lets us reuse the
intermediate multiplication result of the parent length, since we need
to know it to perform our memset(). The result is a little easier to
read.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>