list_objects_filter_options: plug leak of filter_spec strings

The list_objects_filter_options struct contains a string_list to store
the filter_spec. Because we allow the options struct to be
zero-initialized by callers, the string_list's strdup_strings field is
generally not set.

Because we don't want to depend on the memory lifetimes of any strings
passed in to the list-objects API, everything we add to the string_list
is duplicated (either via xstrdup(), or via strbuf_detach()). So far so
good, but now we have a problem at cleanup time: when we clear the
list, the string_list API doesn't realize that it needs to free all of
those strings, and we leak them.

One option would be to set strdup_strings right before clearing the
list. But this is tricky for two reasons:

  1. There's one spot, in partial_clone_get_default_filter_spec(),
     that fails to duplicate its argument. We could fix that, but...

  2. We clear the list in a surprising number of places. As you might
     expect, we do so in list_objects_filter_release(). But we also
     clear and rewrite it in expand_list_objects_filter_spec(),
     list_objects_filter_spec(), and transform_to_combine_type().
     We'd have to put the same hack in all of those spots.

Instead, let's just set strdup_strings before adding anything. That lets
us drop the extra manual xstrdup() calls, fixes the spot mentioned
in (1) above that _should_ be duplicating, and future-proofs further
calls. We do have to switch the strbuf_detach() calls to use the nodup
form, but that's an easy change, and the resulting code more clearly
shows the expected ownership transfer.

This also resolves a weird inconsistency: when we make a deep copy with
list_objects_filter_copy(), it initializes the copy's filter_spec with
string_list_init_dup(). So the copy frees its string_list memory
correctly, but accidentally leaks the extra manual-xstrdup()'d strings!

There is one hiccup, though. In an ideal world, everyone would allocate
the list_objects_filter_options struct with an initializer which used
STRING_LIST_INIT_DUP under the hood. But there are a bunch of existing
callers which think that zero-initializing is good enough. We can leave
them as-is by noting that the list is always initially populated via
parse_list_objects_filter(). So we can just initialize the
strdup_strings flag there.

This is arguably a band-aid, but it works reliably. And it doesn't make
anything harder if we want to switch all the callers later to a new
LIST_OBJECTS_FILTER_INIT.

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

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