object-name: re-use "struct strbuf" in show_ambiguous_object()

object-name: re-use "struct strbuf" in show_ambiguous_object()

Reduce the allocations done by show_ambiguous_object() by moving the
"desc" strbuf into the "struct ambiguous_output" introduced in the
preceding commit.

This doesn't matter for optimization purposes, but since we're
accumulating a "struct strbuf advice" anyway let's follow that pattern
and add a "struct strbuf sb", we can then strbuf_reset() it rather
than calling strbuf_release() for each call to
show_ambiguous_object().

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