Noticed that in one of my git-annex repositories, git-annex was spending
a full second at startup checking all the git-annex branches from remotes
to see if they contained changes that needed to be merged in. So, I added a
cache of recently merged branches to avoid that. I remember considering
this optimisation years ago; don't know why I didn't do it then. Not every
day that I can speed up git-annex so much!

Also, made `git annex log --all` show location log changes for all keys.
This was tricky to get right and fast.