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.

# utility to setup pythonpath to point into the tested repository

export SRCDIR=`dirname $TESTDIR`
if [ -n "$PYTHONPATH" ]; then
    export HGTEST_ORIG_PYTHONPATH=$PYTHONPATH
    export PYTHONPATH=$SRCDIR:$PYTHONPATH
else
    export PYTHONPATH=$SRCDIR
fi