files: use current revision as default for 'show at' (Issue #31)

Before, clicking 'show at' before selecting a changeset would fail when looking
for 'no revision'.

kallithea/controllers/files.py

@@ -605,8 +605,8 b' class FilesController(BaseRepoController'
     def diff(self, repo_name, f_path):
         ignore_whitespace = request.GET.get('ignorews') == '1'
         line_context = request.GET.get('context', 3)
-        diff1 = request.GET.get('diff1', '')
         diff2 = request.GET.get('diff2', '')
+        diff1 = request.GET.get('diff1', '') or diff2
         c.action = request.GET.get('diff')
         c.no_changes = diff1 == diff2
         c.f_path = f_path