ui: provide a mechanism to track and log blocked time We want to log the time Mercurial spends trapped in things outside programmatic control. Provide a mechanism to give us both command runtime and as many different sources of blocking as we deem useful.

mercurial: switch to util.timer for all interval timings util.timer is now the best available interval timer, at the expense of not having a known epoch. Let's use it whenever the epoch is irrelevant.