metaedit: don't change commit date by default (issue5994)

We deliberately unset devel.default-date to allow hg commands to see the real
system date, that demonstrates that metaedit doesn't fall back to system date
for any of these commands.

--HG--
branch : stable
2 jobs for topic/stable/metaedit-date in 11 minutes and 9 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed flake8 #44695

00:00:13

passed tests #44696

00:11:08