util: make strdate's defaults default value a dict

It was specified to be an empty list in c6adf2be6069 in 2007.
It was correct at the time. But when the function was
refactored in 91bc001a592f (2010), it started expecting a dict.
I guess this code path is untested?

Thanks to Yuya for spotting this.

mercurial/util.py

@@ -1830,7 +1830,7 b' def parsetimezone(s):'
 def strdate(string, format, defaults=None):
     """parse a localized time string and return a (unixtime, offset) tuple.
     if the string cannot be parsed, ValueError is raised."""
-    defaults = defaults or []
+    defaults = defaults or {}
 
     # NOTE: unixtime = localunixtime + offset
     offset, date = parsetimezone(string)