Commit
41124a91e61fc6d9681c1e8b15ba30494e84d643 allowed the
transaction log file prefix to be changed but left in place the
existing 64-character limit on the total length of a log file name.
It's possible that could be inconvenient for somebody, so increase the
limit to MAXPGPATH, which ought to be enough for anybody.
Per a suggestion from Tom Lane.