sparse-index: count lstat() calls

sparse-index: count lstat() calls

The clear_skip_worktree.. methods already report some statistics about
how many cache entries are checked against path_found() due to having
the skip-worktree bit set. However, due to path_found() performing some
caching, this isn't the only information that would be helpful to
report.

Add a new lstat_count member to the path_found_data struct to count the
number of times path_found() calls lstat(). This will be helpful to help
explain performance problems in this method as well as to demonstrate
future changes to the caching algorithm in a more concrete way than
end-to-end timings.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>