Fix new completion with web-history-max-items set to 0

We get no last_atime limit at all otherwise:

qutebrowser.misc.sql.SqlException: Failed to prepare query "SELECT url, title,
strftime('%Y-%m-%d', last_atime, 'unixepoch', 'localtime') FROM
CompletionHistory WHERE (url LIKE :pat escape '\' or title LIKE :pat escape '\')
AND last_atime >= ORDER BY last_atime DESC": "near "ORDER": syntax error Unable
to execute statement"

qutebrowser/completion/models/histcategory.py

@@ -60,7 +60,7 @@ class HistoryCategory(QSqlQueryModel):
     def _atime_expr(self):
         """If max_items is set, return an expression to limit the query."""
         max_items = config.get('completion', 'web-history-max-items')
-        if max_items < 0:
+        if max_items <= 0:
             return ''
 
         min_atime = sql.Query(' '.join([
@@ -83,9 +83,10 @@ class HistoryCategory(QSqlQueryModel):
         # treat spaces as wildcards to match any of the typed words
         pattern = re.sub(r' +', '%', pattern)
         pattern = '%{}%'.format(pattern)
-        with debug.log_time('sql', 'Running completion query'):
-            self._query.run(pat=pattern)
-        self.setQuery(self._query)
+        if config.get('completion', 'web-history-max-items') != 0:
+            with debug.log_time('sql', 'Running completion query'):
+                self._query.run(pat=pattern)
+            self.setQuery(self._query)
 
     def delete_cur_item(self, index):
         """Delete the row at the given index."""

tests/unit/completion/test_histcategory.py

@@ -117,7 +117,12 @@ def test_set_pattern(pattern, before, after, model_validator, hist):
     ], [
         ('b', 'b', '2017-06-16'),
         ('c', 'c', '2017-05-16'),
-    ])
+    ]),
+    (0, [
+        ('a', 'a', '2017-04-16'),
+        ('b', 'b', '2017-06-16'),
+        ('c', 'c', '2017-05-16'),
+    ], []),
 ])
 def test_sorting(max_items, before, after, model_validator, hist, config_stub):
     """Validate the filtering and sorting results of set_pattern."""