ls-tree: use "size_t", not "int" for "struct strbuf"'s "len"

ls-tree: use "size_t", not "int" for "struct strbuf"'s "len"

The "struct strbuf"'s "len" member is a "size_t", not an "int", so
let's change our corresponding types accordingly. This also changes
the "len" and "speclen" variables, which are likewise used to store
the return value of strlen(), which returns "size_t", not "int".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>