blackbox: change year in logs to ISO 8601 format AFAIK, year/month/day is not a format than anyone uses. It seems more sensible to me to use ISO 8601, as that's standard and unambiguously year-month-day. Compatibility-wise, I think it's acceptable to change the default format. It's most for human consumption. It's plausible that a few tools parse this format, but it has already changed in the past (for instance to add the current revision). Differential Revision: https://phab.mercurial-scm.org/D12006

blackbox: add milliseconds to blackbox logs by default The current second granularity is often not specific enough to determine whether an hg command is happening before or after some other event. Given that starting a process takes on the order of 1ms (well, for native processes. It's quite a bit more for python processes), microseconds seems like unnecessary noise. This also lines up behavior with the rust version, where we already switched to millisecond precision. Differential Revision: https://phab.mercurial-scm.org/D12005

blackbox: correct date format doc Differential Revision: https://phab.mercurial-scm.org/D12004