parse-options API: remove OPTION_ARGUMENT feature

As was noted in 1a85b49b87a (parse-options: make OPT_ARGUMENT() more useful, 2019-03-14) there's only ever been one user of the OPT_ARGUMENT(), that user was added in 20de316e334 (difftool: allow running outside Git worktrees with --no-index, 2019-03-14). The OPT_ARGUMENT() feature itself was added way back in 580d5bffdea (parse-options: new option type to treat an option-like parameter as an argument., 2008-03-02), but as discussed in 1a85b49b87a wasn't used until 20de316e334 in 2019. Now that the preceding commit has migrated this code over to using "struct strvec" to manage the "args" member of a "struct child_process", we can just use that directly instead of relying on OPT_ARGUMENT. This has a minor change in behavior in that if we'll pass --no-index we'll now always pass it as the first argument, before we'd pass it in whatever position the caller did. Preserving this was the real value of OPT_ARGUMENT(), but as it turns out we didn't need that either. We can always inject it as the first argument, the other end will parse it just the same. Note that we cannot remove the "out" and "cpidx" members of "struct parse_opt_ctx_t" added in 580d5bffdea, while they were introduced with OPT_ARGUMENT() we since used them for other things. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>