Small improvements for allocation logic in ginHeapTupleFastCollect().

Small improvements for allocation logic in ginHeapTupleFastCollect().

Avoid repetitive calls to repalloc() when the required size of the
collector array grows more than 2x in one call.  Also ensure that the
array size is a power of 2 (since palloc will probably consume a power
of 2 anyway) and doesn't start out very small (which'd likely just lead
to extra repallocs).

David Rowley, tweaked a bit by me

Discussion: https://postgr.es/m/CAKJS1f8vn-iSBE8PKeVHrnhvyjRNYCxguPFFY08QLYmjWG9hPQ@mail.gmail.com