Re: [PATCH] git-submodule.sh: Support 'checkout' as a valid update command

> It is not about preference but what we want to convey to the
> readers.  When you start the sentence with "Oh, it already works
> correctly", the readers need to see this sentence finished: "It
> already works, it is handled correctly, but we change the code
> nevertheless because ...?".
>
> Here is my attempt to fill that "because ..." part:
>
>         Subject: git-submodule.sh: 'checkout' is a valid update mode
>
>         'checkout' is documented as one of the valid values for
>         'submodule.<name>.update' variable, and in a repository with
>         the variable set to 'checkout', "git submodule update"
>         command do update using the 'checkout' mode.
>
>         However, it has been an accident that the implementation
>         works this way; any unknown value would trigger the same
>         codepath and update using the 'checkout' mode.
>
>         Tighten the codepath and explicitly list 'checkout' as one
>         of the known update modes, and error out when an unknown
>         update mode is used.
>
>         Also, teach the codepath that initializes the configuration
>         variable from in-tree .gitmodules that 'checkout' is one of
>         the valid values---the code since ac1fbbda (submodule: do
>         not copy unknown update mode from .gitmodules, 2013-12-02)
>         used to treat the value 'checkout' as unknown and mapped it
>         to 'none', which made little sense.
>