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

hgext/histedit.py

@@ -1151,7 +1151,7 b' class histeditrule(object):'
             h,
         )
 
-    @property
+    @util.propertycache
     def desc(self):
         summary = (
             cmdutil.rendertemplate(