index: add trace2 region for clear skip worktree

index: add trace2 region for clear skip worktree

When using sparse checkout, clear_skip_worktree_from_present_files() must
enumerate index entries to find ones with the SKIP_WORKTREE bit to
determine whether those index entries exist on disk (in which case their
SKIP_WORKTREE bit should be removed).

In a large repository, this may take considerable time depending on the
size of the index.

Add a trace2 region to surface this information, keeping a count of how
many paths have been checked. Separately, keep counts after a full index is
materialized.

Signed-off-by: Anh Le <anh@canva.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>