difftool docs: de-duplicate configuration sections

Include the "config/difftool.txt" file in "git-difftool.txt", and move
the relevant part of git-difftool(1) configuration from
"config/diff.txt" to config/difftool.txt".

Doing this is slightly odd, as we usually discuss configuration in
alphabetical order, but by doing it we're able to include the full set
of configuration used by git-difftool(1) (and only that configuration)
in its own documentation.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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