Fix for 823431: pt-query-advisor hangs on big queries

This was caused by a regex backtracking itself to a halt. The solution
was to add a positive lookahead that searches for the floating but mandatory
substrings that the regex needs; if those exist, then eventually it will
match, but if they don't, it'll fail quickly.

bin/pt-query-advisor

@@ -4490,6 +4490,7 @@ sub parse_insert {
    }
 
    if ( my @into = ($query =~ m/
+            (?=VALUE|SE(?:T|LECT))  # Avoid a backtracking explosion
             (?:INTO\s+)?            # INTO, optional
             (.+?)\s+                # table ref
             (\([^\)]+\)\s+)?        # column list, optional