ls-tree: make "line_termination" less generic

ls-tree: make "line_termination" less generic

The "ls-tree" command isn't capable of ending "lines" with anything
except '\n' or '\0', and in the latter case we can avoid calling
write_name_quoted_relative() entirely. Let's do that, less for
optimization and more for clarity, the write_name_quoted_relative()
API itself does much the same thing.

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