view CONTRIBUTING @ 45450:47d10ade5bc4

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

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