Differentiate MERGE queries with different structures

Hi,

pg_stat_statements module distinguishes queries with different 
structures, but some visibly different MERGE queries were combined as 
one pg_stat_statements entry.
For example,
MERGE INTO test1 USING test2 ON test1.id = test2.id WHEN MATCHED THEN 
UPDATE var = 1;
MERGE INTO test1 USING test2 ON test1.id = test2.id WHEN MATCHED THEN 
DELETE;
These two queries have different command types after WHEN (UPDATE and 
DELETE), but they were regarded as one entry in pg_stat_statements.
I think that they should be sampled as distinct queries.

I attached a patch file that adds information about MERGE queries on the 
documentation of pg_stat_statements, and lines of code that helps with 
the calculation of queryid hash value to differentiate MERGE queries.
Any kind of feedback is appreciated.

Tatsu