In rewriteheap.c (used by VACUUM FULL and CLUSTER), calculate the tuple

In rewriteheap.c (used by VACUUM FULL and CLUSTER), calculate the tuple
length stored in the line pointer the same way it's calculated in the normal
heap_insert() codepath. As noted by Jeff Davis, the length stored by
raw_heap_insert() included padding but the one stored by the normal codepath
did not. While the mismatch seems to be harmless, inconsistency isn't good,
and the normal codepath has received a lot more testing over the years.

Backpatch to 8.3 where the heap rewrite code was introduced.