util: use single quotes in use warning

mercurial/util.py

@@ -1953,7 +1953,7 b' def matchdate(date):'
         except ValueError:
             raise Abort(_("invalid day spec: %s") % date[1:])
         if days < 0:
-            raise Abort(_('%s must be nonnegative (see "hg help dates")')
+            raise Abort(_("%s must be nonnegative (see 'hg help dates')")
                 % date[1:])
         when = makedate()[0] - days * 3600 * 24
         return lambda x: x >= when

tests/test-parse-date.t

@@ -102,7 +102,7 b' Normal range'
 Negative range
 
   $ hg log -d "--2"
-  abort: -2 must be nonnegative (see "hg help dates")
+  abort: -2 must be nonnegative (see 'hg help dates')
   [255]
 
 Whitespace only