list-objects-filter-options: make filter_spec a string_list

list-objects-filter-options: make filter_spec a string_list

Make the filter_spec string a string_list rather than a raw C string.
The list of strings must be concatted together to make a complete
filter_spec. A future patch will use this capability to build "combine:"
filter specs gradually.

A strbuf would seem to be a more natural choice for this object, but it
unfortunately requires initialization besides just zero'ing out the
memory.  This results in all container structs, and all containers of
those structs, etc., to also require initialization. Initializing them
all would be more cumbersome that simply using a string_list, which
behaves properly when its contents are zero'd.

For the purposes of code simplification, change behavior in how filter
specs are conveyed over the protocol: do not normalize the tree:<depth>
filter specs since there should be no server in existence that supports
tree:# but not tree:#k etc.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>