Merge branch 'dw/mergetool-vim-window-shuffle' into maint

The vimdiff backend for "git mergetool" has been tweaked to arrange
and number buffers in the order that would match the expectation of
majority of people who read left to right, then top down and assign
buffers 1 2 3 4 "mentally" to local base remote merge windows based
on that order.

* dw/mergetool-vim-window-shuffle:
  mergetool: reorder vim/gvim buffers in three-way diffs