mergetool-lib: add a three-way diff view for vim/gvim

When the base version is available, use a three-way, four panel view by
default. This shows the (local, base, remote) revisions up top and the
merged result by itself in the lower pane. All revisions will still scroll
together by default, and the cursor still defaults to the merged result edit
pane.

Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -172,8 +172,13 @@ run_merge_tool () {
 	vimdiff|gvimdiff)
 		if merge_mode; then
 			touch "$BACKUP"
-			"$merge_tool_path" -f -d -c "wincmd l" \
-				"$LOCAL" "$MERGED" "$REMOTE"
+			if $base_present; then
+				"$merge_tool_path" -f -d -c "wincmd J" \
+					"$MERGED" "$LOCAL" "$BASE" "$REMOTE"
+			else
+				"$merge_tool_path" -f -d -c "wincmd l" \
+					"$LOCAL" "$MERGED" "$REMOTE"
+			fi
 			check_unchanged
 		else
 			"$merge_tool_path" -f -d -c "wincmd l" \