I try to follow the BLUF pattern: Bottom line up front. The first line is as short a description of the change ("Re-fixed a bug where a URL without a verb could crash the bot.") with some detail following ("I thought I caught that a couple of years back...")

I try to save the detail for the code itself: Comments describe what I was thinking at the time for context, the code is the code. I don't replicate the code comments in the commit message because having the same thing in two places means having to keep two things up to date, and that rarely goes well.