De-kludge contrib/btree_gin for collations.

Using DEFAULT_COLLATION_OID in the comparePartial functions was not only
a lame hack, but outright wrong, because the compare functions for
collation-aware types were already responding to the declared index
collation.  So comparePartial would have the wrong expectation about
the index's sort order, possibly leading to missing matches for prefix
searches.

contrib/btree_gin/btree_gin.c

@@ -7,7 +7,6 @@
 
 #include "fmgr.h"
 #include "access/skey.h"
-#include "catalog/pg_collation.h"
 #include "utils/builtins.h"
 #include "utils/bytea.h"
 #include "utils/cash.h"
@@ -123,7 +122,7 @@ gin_compare_prefix_##type(PG_FUNCTION_ARGS)									\
 																			\
 	cmp = DatumGetInt32(DirectFunctionCall2Coll(							\
 				TypeInfo_##type.typecmp,									\
-				DEFAULT_COLLATION_OID,										\
+				PG_GET_COLLATION(),											\
 				(data->strategy == BTLessStrategyNumber ||					\
 				 data->strategy == BTLessEqualStrategyNumber)				\
 				 ? data->datum : a,											\