mergetool: reorder vim/gvim buffers in three-way diffs

When invoking default (g)vimdiff three-way merge, the merged file is
loaded as the first buffer but moved to the bottom as the fourth window.
This causes a disconnect between vim commands that operate on window
positions (e.g. CTRL-W_w) and those that operate on buffer index (e.g.
do/dp).

This change reorders the buffers to have the same index as windows while
keeping the cursor default to the merged result as the bottom window.

Signed-off-by: Dickson Wong <dicksonwong@gmail.com>
Tested-by: Michael J Gruber <git@drmicha.warpmail.net>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/vimdiff

@@ -9,8 +9,8 @@ merge_cmd () {
 	gvimdiff|vimdiff)
 		if $base_present
 		then
-			"$merge_tool_path" -f -d -c 'wincmd J' \
-				"$MERGED" "$LOCAL" "$BASE" "$REMOTE"
+			"$merge_tool_path" -f -d -c '4wincmd w | wincmd J' \
+				"$LOCAL" "$BASE" "$REMOTE" "$MERGED"
 		else
 			"$merge_tool_path" -f -d -c 'wincmd l' \
 				"$LOCAL" "$MERGED" "$REMOTE"