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