perf: add perfprogress command I've noticed that progress bars can add significant overhead to tight loops. Let's add a perf command that attempts to isolate that overhead. With a default hgrc, iteration over 1M items appears to take ~3.75s on my machine. Profiling reveals ~28% of time is spent in ui.configbool() resolving the value of the progress.debug config option. Even if I set progress.disable=true, execution still takes ~2.60s, with ~59% of the time spent in ui.configbool(). Differential Revision: https://phab.mercurial-scm.org/D5407