Auto-gc in the background can take a long time to be put in the background

Hi,

Recently, I've noticed that whenever the auto-gc message shows up about being spawned in the background, it still takes a while for git to return to the shell.

I've finally looked at what it was stuck on, and it's `git reflog expire --all` taking more than 30s. I guess the question is whether there's a reason this shouldn't run in the background? Another is whether there's something that makes this slower than it should be.

Mike