git-checkout doesn't seem to respect config from include.path

Hi,

Not sure if this is confined specifically to `git-checkout`, but that's the command I noticed the issue with:

With the release of the v2.35.0 and the "zdiff3" setting for "merge.conflictStyle", I find myself wanting to use "zdiff3" on machines running the new version, and falling back to "diff3" on machines with an older version.

To this end, I have a ~/.gitconfig that contains:

    [merge]
    	conflictStyle = zdiff3
    [include]
    	path = ~/.gitconfig.local

The idea is that I can use the same `~/.gitconfig` on every machine I use, but on machines that only have an older Git version, I drop in a ~/.gitconfig.local with overrides like this:

    [merge]
    	conflictStyle = diff3

`git config --get merge.conflictStyle` correctly reports that my setting is "diff3" on such machines, and `git config --get-all merge.conflictStyle` shows:

    diff3
    zdiff3

In other words, it knows that I have multiple values set, but it uses a last-one-wins policy.

However, when I try to run a command like `git checkout -b something`, Git dies with:

    fatal: unknown style 'zdiff3' given for 'merge.conflictstyle'

So, it looks like something in `git-checkout`'s option processing is causing it to disregard the override set via "include.path". In fact, it even disregards a value passed in with `-c` like this:

    git -c merge.conflictStyle=diff3 checkout -b something

Does this sound like a bug, or are my expectations off? I'd be happy to look into fixing this, but first would like to know whether it is expected behavior.

Cheers, Greg