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>