Fix error caused on some boxes by using diff when it's not defined.

bin/pt-mysql-summary

@@ -2421,7 +2421,7 @@ main() {
 # Execute the program if it was not included from another file.
 # This makes it possible to include without executing, and thus test.
 if    [ "${0##*/}" = "$TOOL" ] \
-   || [ "${0##*/}" = "bash" -a "$_" = "$0" ]; then
+   || [ "${0##*/}" = "bash" -a "${_:-""}" = "$0" ]; then
 
    # Set up temporary dir.
    mk_tmpdir