When you call "git reset --mixed <paths>" git will warn that using mixed
with paths is deprecated:

    warning: --mixed option is deprecated with paths.

That doesn't tell the user what he should use instead. Expand on the
warning and tell the user to just omit --mixed:

    warning: --mixed with paths is deprecated; use 'git reset -- <paths>' instead

The exact wording of the warning was suggested by Jonathan Nieder.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>