fix: keep RENAME TABLE clauses as separate identifiers (issue779)

RENAME TABLE a TO b, c TO d grouped the target of the first clause and
the source of the second into a single IdentifierList, because the comma
between the two clauses was treated as an identifier-list separator.

Detect the rename layout by the TO keywords flanking both sides of the
comma and leave such a comma ungrouped, so each renamed table stays a
separate Identifier. Genuine target lists such as GRANT ... TO a, b are
left untouched.

sqlparse/engine/grouping.py

@@ -309,13 +309,34 @@ def group_operator(tlist):
 
 def group_identifier_list(tlist):
     m_role = T.Keyword, ('null', 'role')
+    m_to = T.Keyword, 'TO'
     sqlcls = (sql.Function, sql.Case, sql.Identifier, sql.Comparison,
               sql.IdentifierList, sql.Operation)
     ttypes = (T_NUMERICAL + T_STRING + T_NAME
               + (T.Keyword, T.Comment, T.Wildcard))
 
+    def is_rename_separator(token):
+        # In ``RENAME TABLE a TO b, c TO d`` the comma separates independent
+        # rename clauses, so the target of one clause (``b``) and the source
+        # of the next (``c``) must not be merged into a single IdentifierList.
+        # Detect that layout by the ``TO`` keywords flanking both sides of the
+        # comma and leave such a comma ungrouped (issue779).
+        parent = token.parent
+        if parent is None:
+            return False
+        cidx = parent.token_index(token)
+        pidx, prev_ = parent.token_prev(cidx)
+        nidx, next_ = parent.token_next(cidx)
+        if prev_ is None or next_ is None:
+            return False
+        _, before_prev = parent.token_prev(pidx)
+        _, after_next = parent.token_next(nidx)
+        return (before_prev is not None and before_prev.match(*m_to)
+                and after_next is not None and after_next.match(*m_to))
+
     def match(token):
-        return token.match(T.Punctuation, ',')
+        return token.match(T.Punctuation, ',') \
+            and not is_rename_separator(token)
 
     def valid(token):
         return imt(token, i=sqlcls, m=m_role, t=ttypes)