Make transpose-regions invoke before-change-functions only once.

Make transpose-regions invoke before-change-functions only once.

In the case of two non-contiguous regions the same size, transpose-regions has
been calling before-change-functions twice, once for each region.  It now
calls it just once, for the minimal region spanning both single regions.

* src/editfns.c (Ftranspose_regions): Combine two calls of modify_text into
one.