tests: fix vcs Hg helper function get_config_value to not modify global state when using a custom config file

This leak was what made test_get_user_name and test_get_user_email pass when
test_get_config_value had been run first.

kallithea/lib/vcs/backends/hg/repository.py

@@ -602,8 +602,10 b' class MercurialRepository(BaseRepository'
             config_file = [config_file]
 
         config = self._repo.ui
-        for path in config_file:
-            config.readconfig(path)
+        if config_file:
+            config = ui.ui()
+            for path in config_file:
+                config.readconfig(path)
         return config.config(section, name)
 
     def get_user_name(self, config_file=None):