Merge branch 'cj/p4merge' into maint

* cj/p4merge:
  Pass empty file to p4merge where no base is suitable.

git-mergetool--lib.sh

@@ -259,11 +259,8 @@ run_merge_tool () {
 	p4merge)
 		if merge_mode; then
 			touch "$BACKUP"
-			if $base_present; then
+			$base_present || >"$BASE"
 			"$merge_tool_path" "$BASE" "$LOCAL" "$REMOTE" "$MERGED"
-			else
-				"$merge_tool_path" "$LOCAL" "$LOCAL" "$REMOTE" "$MERGED"
-			fi
 			check_unchanged
 		else
 			"$merge_tool_path" "$LOCAL" "$REMOTE"