test: fix test-evolve.t Metaedit take the current date when no date is given. When we added default- date, the date ended up to be the same and the behavior changed. Fix the text by disabling default-date in the metaedit call and update the comment about what is needed to fix correctly the test; take the changeset date when metaediting only one changeset.

#require test-repo pyflakes

Copied from Mercurial core (60ee2593a270)

  $ cd "`dirname "$TESTDIR"`"

run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)

  $ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
  > | xargs pyflakes 2>/dev/null