LP:873142 - GREATEST() does not always return same signness of argument types.

Description:
There is a problem on all platforms; to varying degrees.

Windows 32bit and Windows 64bit:
When supplied BIGINT UNSIGNED, the return type is a BIGINT SIGNED,

Others:
The return type appears to be valid however for BIGINT values, the comparison is always
performed as if the values were signed.

Workaround would be to use IF(x>y,x,y) function instead.

How to repeat:
To reproduce this bug, create this simple table.

DROP TABLE `cma`;

CREATE TABLE `cma` (

  `a` binary(16) NOT NULL,

  `b` bigint(20) unsigned NOT NULL,

  PRIMARY KEY (`a`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- Execute the following statement twice

INSERT INTO cma

(a, b)

VALUES ('foobar',13836376518955650385)

ON DUPLICATE KEY UPDATE

b=GREATEST(b,VALUES(b));