The config include mechanism doesn't allow for overwriting

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

I was hoping to write something like this:

    [user]
        name = Luser
        email = some-default@example.com
    [include]
        path = ~/.gitconfig.d/user-email

Where that file would contain:

    [user]
        email = local-email@example.com

But when you do that git prints:

    $ git config --get user.email
     some-default@example.com
     error: More than one value for the key user.email: local-email@example.com

I couldn't find information in either the commt that introduced the
feature or the documentation explaining whether this was the intent or
not.

I think config inclusion is much less useful when you can't clobber
previously assigned values.