devel: add a config field to force dates to timestamp 0

Add a new config field named default-date under the devel section to force all
implicits date to a specific value. If a explicit date is passed, it will
override the default.

This patch only affect changesets. Other usages (blackbox, obsmarkers) are
updated in later patchs.

The test runner is setting a bunch of alias to force the '--date' argument. We
will replace theses aliases in a later patch.

mercurial/context.py

@@ -1353,7 +1353,11 b' class committablectx(basectx):'
 
     @propertycache
     def _date(self):
-        return util.makedate()
+        ui = self._repo.ui
+        date = ui.configdate('devel', 'default-date')
+        if date is None:
+            date = util.makedate()
+        return date
 
     def subrev(self, subpath):
         return None