histedit: cache description line Navigating the curses-based histedit interface can be pretty slow because it redraws everything whenever you make a change. This patch simply replaces `@property` by `@util.propertycache` on the `histeditrule.desc()` function so it's not re-calculated every time the screen needs to be re-rendered. I timed it on an example of 30 simple commits, where I moved the top commit down 25 steps and then up 25 steps after. Before this patch, that (the whole `hg histedit` invocation) took 11.6 s of CPU and after this patch it took 0.8 s). Differential Revision: https://phab.mercurial-scm.org/D9016