porters-handbook: Use full-length hashes in GH_TAGNAME

Full hashes avoid possible conflicts, at the cost of a small increase in
some path names.

About 1/3 of the existing GH_TAGNAMEs in the ports tree are already full
40-character hashes.

Reviewed by: bcr
Event: Halifax Hackathon 202606
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57901