histedit: log the time taken to read in the commands list

If we're being fed an external command list from stdin (histedit --commands -),
then the time spent reading stdin is outside our control. Log it.

hgext/histedit.py

@@ -992,7 +992,8 b' def _getgoal(opts):'
 
 def _readfile(ui, path):
     if path == '-':
-        return ui.fin.read()
+        with ui.timeblockedsection('histedit'):
+            return ui.fin.read()
     else:
         with open(path, 'rb') as f:
             return f.read()