trace2: add region in clear_ce_flags

trace2: add region in clear_ce_flags

When Git updates the working directory with the sparse-checkout
feature enabled, the unpack_trees() method calls clear_ce_flags()
to update the skip-wortree bits on the cache entries. This
check can be expensive, depending on the patterns used.

Add trace2 regions around the method, including some flag
information, so we can get granular performance data during
experiments. This data will be used to measure improvements
to the pattern-matching algorithms for sparse-checkout.

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