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