Problem:
--------
When global transaction identifiers (GTIDs) were
implemented, a working name sometimes used internally was
'group' rather than 'transaction'. This was rejected and we
now use 'transaction' everywhere. But there are still
remainders of 'group' in the code. So we should rename some
C++ identifiers.

Note: this has no user-visible effects, it only changes C++
identifier names.

Fix:
----
Renamed,

enum_group_type
{ AUTOMATIC_GROUP, ANONYMOUS_GROUP, INVALID_GROUP,
  UNDEFINED_GROUP,GTID_GROUP, NOT_YET_DETERMINED_GROUP },
ER_GTID_NEXT_TYPE_UNDEFINED_GROUP,
ER_GTID_UNSAFE_BINLOG_SPLITTABLE_STATEMENT_AND_GTID_GROUP

to

enum_gtid_type
{ AUTOMATIC_GTID, ANONYMOUS_GTID, INVALID_GTID,
  UNDEFINED_GTID, ASSIGNED_GTID, NOT_YET_DETERMINED_GTID },
ER_GTID_NEXT_TYPE_UNDEFINED_GTID,
ER_GTID_UNSAFE_BINLOG_SPLITTABLE_STATEMENT_AND_ASSIGNED_GTID.