list-objects-filter-options: support 'auto' mode for --filter

list-objects-filter-options: support 'auto' mode for --filter

In a following commit, we are going to allow passing "auto" as a
<filterspec> to the `--filter=<filterspec>` option, but only for some
commands. Other commands that support the `--filter=<filterspec>`
option should still die() when 'auto' is passed.

Let's set up the "list-objects-filter-options.{c,h}" infrastructure to
support that:

- Add a new `unsigned int allow_auto_filter : 1;` flag to
  `struct list_objects_filter_options` which specifies if "auto" is
  accepted or not by the current command.
- Change gently_parse_list_objects_filter() to parse "auto" if it's
  accepted.
- Make sure we die() if "auto" is combined with another filter.
- Update list_objects_filter_release() to preserve the
  allow_auto_filter flag, as this function is often called (via
  opt_parse_list_objects_filter) to reset the struct before parsing a
  new value.

Let's also update `list-objects-filter.c` to recognize the new
`LOFC_AUTO` choice. Since "auto" must be resolved to a concrete filter
before filtering actually begins, initializing a filter with
`LOFC_AUTO` is invalid and will trigger a BUG().

Note that ideally combining "auto" with "auto" could be allowed, but in
practice, it's probably not worth the added code complexity. And if we
really want it, nothing prevents us to allow it in future work.

If we ever want to give a meaning to combining "auto" with a different
filter too, nothing prevents us to do that in future work either.

Also note that the new `allow_auto_filter` flag depends on the command,
not user choices, so it should be reset to the command default when
`struct list_objects_filter_options` instances are reset.

While at it, let's add a new "u-list-objects-filter-options.c" file for
`struct list_objects_filter_options` related unit tests. For now it
only tests gently_parse_list_objects_filter() though.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>