For a few days, it looks like build.log.gz contains uncompressed text, and build.log the gzipped version. For example: https://copr-be.cloud.fedoraproject.org/results/nalimilan/julia-nightlies/fedora-21-x86_64/pcre2-10.10-3.fc23/root.log.gz
Ah, seen the duplicate report right after clicking on Submit. *** This bug has been marked as a duplicate of bug 1223670 ***