magit-blame: fix previous way

As this function is called by magit-blame-previous-chunk, it needs to
be able to inverse the search direction.
We use the order of beg/end args to do this:
   - if beg > end, then return the previous chunk
   - if beg < end, then return the next chunk

Signed-off-by: Sylvain Chouleur <sylvain.chouleur@gmail.com>