This is mostly just a personal quality-of-life fix. I run this command fairly often and having it return a little faster is nice.
This replaces a git show for each individual branch with a big git for-each-ref which we were already running anyway. This is quite a bit faster.
This command also occasionally hangs or segfaults for me while executing the huge pile of subprocesses. This is unreliable to reproduce, probably some bug in some PHP extension I have, and likely hard to narrow down, and this approach is better in every way anyway.