This needs a small tweak to work on macOS, where git uses the POSIX version of grep (which doesn't support `\b`). You need to use the Perl Regexp option by switching -E with -P:

git log -i -P --grep="\b(fix|fixed|fixes|bug|broken)\b" --name-only --format='' | sort | uniq -c | gsort -nr | head -20