Rename heapam_index_fetch_tuple argument for clarity.

Rename heapam_index_fetch_tuple argument for clarity.

Rename heapam_index_fetch_tuple's call_again argument to heap_continue,
for consistency with the pointed-to variable name (IndexScanDescData's
xs_heap_continue field).

Preparation for an upcoming commit that will move index scan related
heapam functions into their own file.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/bmbrkiyjxoal6o5xadzv5bveoynrt3x37wqch7w3jnwumkq2yo@b4zmtnrfs4mh