sparse-checkout: be consistent with end of options markers

sparse-checkout: be consistent with end of options markers

93851746 (parse-options: decouple "--end-of-options" and "--",
2023-12-06) updated the world order to make callers of parse-options
that set PARSE_OPT_KEEP_UNKNOWN_OPT responsible for deciding what to
do with "--end-of-options" they may see after parse_options() returns.

This made a previous bug in sparse-checkout more visible; namely,
that

  git sparse-checkout [add|set] --[no-]cone --end-of-options ...

would simply treat "--end-of-options" as one of the paths to include in
the sparse-checkout.  But this was already problematic before; namely,

  git sparse-checkout [add|set| --[no-]cone --sikp-checks ...

would not give an error on the mis-typed "--skip-checks" but instead
simply treat "--sikp-checks" as a path or pattern to include in the
sparse-checkout, which is highly unfriendly.

This behavior began when the command was converted to parse-options in
7bffca95ea (sparse-checkout: add '--stdin' option to set subcommand,
2019-11-21).  Back then it was just called KEEP_UNKNOWN. Later it was
renamed to KEEP_UNKNOWN_OPT in 99d86d60e5 (parse-options:
PARSE_OPT_KEEP_UNKNOWN only applies to --options, 2022-08-19) to clarify
that it was only about dashed options; we always keep non-option
arguments.  Looking at that original patch, both Peff and I think that
the author was simply confused about the mis-named option, and really
just wanted to keep the non-option arguments.  We never should have used
the flag all along (and the other cases were cargo-culted within the
file).

Remove the erroneous PARSE_OPT_KEEP_UNKNOWN_OPT flag now to fix this
bug.  Note that this does mean that anyone who might have been using

  git sparse-checkout [add|set] [--[no-]cone] --foo --bar

to request paths or patterns '--foo' and '--bar' will now have to use

  git sparse-checkout [add|set] [--[no-]cone] -- --foo --bar

That makes sparse-checkout more consistent with other git commands,
provides users much friendlier error messages and behavior, and is
consistent with the all-caps warning in git-sparse-checkout.txt that
this command "is experimental...its behavior...will likely change".  :-)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>