use less space in xl_xact_commit patch

Hi,


following the conversation at

http://postgresql.1045698.n5.nabble.com/switch-UNLOGGED-to-LOGGED-tp4290461p4382333.html


I tried to remove some bytes from xl_xact_commit.

The way I did it needs palloc+memcpy. I guess it could be done
reusing the memory for smgrGetPendingDeletes. But I don't
think it's that important.

I guess there are other ways of doing it; let me know what
you think.



Leonardo