test-commit: test unset date is not read as epoch origin

tests/test-commit

@@ -7,6 +7,10 b' echo foo > foo'
 hg add foo
 HGEDITOR=true hg commit -m ""
 hg commit -d '0 0' -m commit-1
+# An empty date was interpreted as epoch origin
+echo foo >> foo
+hg commit -d '' -m commit-no-date
+hg tip --template '{date|isodate}\n' | grep '1970'
 echo foo >> foo
 hg commit -d '1 4444444' -m commit-3
 hg commit -d '1	15.1' -m commit-4