unpack-trees: add trace2 regions

unpack-trees: add trace2 regions

The unpack_trees() method is quite complicated and its performance can
change dramatically depending on how it is used. We already have some
performance tracing regions, but they have not been updated to the
trace2 API. Do so now.

We already have trace2 regions in unpack_trees.c:clear_ce_flags(), which
uses a linear scan through the index without recursing into trees.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>