MDEV-37842 Skip implicit Using_Gtid warning when value is unchanged

Description

When a replica already has Using_Gtid=No and a CHANGE MASTER TO is issued with log coordinates (e.g. relay_log_pos, master_log_file), the server emits a spurious warning:

Note 4190: CHANGE MASTER TO is implicitly changing the value of 'Using_Gtid' from 'No' to 'No'

The value isn't actually changing, so the warning is misleading.

Root Cause

In change_master() (sql/sql_repl.cc), when log coordinates are specified without an explicit master_use_gtid, the code implicitly sets Using_Gtid to No and emits a warning. The condition only checks whether master_use_gtid=No was explicitly given — it does not check whether Using_Gtid is already No. This causes the "from No to No" warning.

Fix

Added a check that the current Using_Gtid value differs from USE_GTID_NO before emitting the warning. The warning now only fires when the value actually changes.

How to test

Start a MariaDB 10.11+ server and connect as a client:

-- Set Using_Gtid to No
CHANGE MASTER TO MASTER_USE_GTID=NO;

-- Before fix: produces "changing from 'No' to 'No'" warning
-- After fix: no warning
CHANGE MASTER TO relay_log_pos=0;
SHOW WARNINGS;

Expected (after fix): Empty result — no warning.

Verify the warning still fires when the value actually changes:

-- Set Using_Gtid to Slave_Pos
CHANGE MASTER TO MASTER_USE_GTID=Slave_Pos;

-- Should produce a warning: changing from 'Slave_Pos' to 'No'
CHANGE MASTER TO relay_log_pos=0;
SHOW WARNINGS;

Expected: Note 4190: CHANGE MASTER TO is implicitly changing the value of 'Using_Gtid' from 'Slave_Pos' to 'No'