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
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