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