better error message when forget to call sub-command

[kurtbrose]

I find myself mistakenly doing this:

executor.git.stash.redirect()

When it should be:

executor.git.stash().redirect()

This happens so often, it would be nice if there was either a consistent way to make it work, or a nice error message. Currently you get something like this AttributeError: 'functools.partial' object has no attribute 'batch'

(This is similar to the other issue about functools.partial AttributeError but a different way to cause the error I think.)