Merge branch 'da/mergetool-diff-order'

"git mergetool" learned to honor "-O<orderfile>" to control the
order of paths to present to the end user.

* da/mergetool-diff-order:
  mergetool: honor -O<orderfile>
  mergetool: honor diff.orderFile
  mergetool: move main program flow into a main() function
  mergetool: add copyright