GIN code managing entry insertion not able to differentiate fresh and old indexes

Hi all,

While playing with the GIN code for an upcoming patch, I noticed that
when inserting a new entry in a new index, this code path is not able
to make the difference if the index is in a build state or not.
Basically, when entering in ginEntryInsert(at)gininsert(dot)c GinBtree built
via ginPrepareEntryScan does not have its flag isBuild set up
properly. I think that it should be set as follows to let this code
path be aware that index is in build state:
btree.isBuild = (buildStats != NULL);

Note that the entry insertion code does nothing with isBuild yet, so
it does not really impact back-branches. However, if in the future we
fix a bug in this area and need to make distinction between a fresh
index and an old one well there will be problems. For those reasons,
this correctness fix should be perhaps master-only for now (perhaps
even 9.4 stuff as well).

Patch is attached.

Regards,
--
Michael