list-objects-filter: add and use initializers

list-objects-filter: add and use initializers

In 7e2619d8ff (list_objects_filter_options: plug leak of filter_spec
strings, 2022-09-08), we noted that the filter_spec string_list was
inconsistent in how it handled memory ownership of strings stored in the
list. The fix there was a bit of a band-aid to set the "strdup_strings"
variable right before adding anything.

That works OK, and it lets the users of the API continue to
zero-initialize the struct. But it makes the code a bit hard to follow
and accident-prone, as any other spots appending the filter_spec need to
think about whether to set the strdup_strings value, too (there's one
such spot in partial_clone_get_default_filter_spec(), which is probably
a possible memory leak).

So let's do that full cleanup now. We'll introduce a
LIST_OBJECTS_FILTER_INIT macro and matching function, and use them as
appropriate (though it is for the "_options" struct, this matches the
corresponding list_objects_filter_release() function).

This is harder than it seems! Many other structs, like
git_transport_data, embed the filter struct. So they need to initialize
it themselves even if the rest of the enclosing struct is OK with
zero-initialization. I found all of the relevant spots by grepping
manually for declarations of list_objects_filter_options. And then doing
so recursively for structs which embed it, and ones which embed those,
and so on.

I'm pretty sure I got everything, but there's no change that would alert
the compiler if any topics in flight added new declarations. To catch
this case, we now double-check in the parsing function that things were
initialized as expected and BUG() if appropriate.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>