read-cache: log the number of scanned files to trace2

read-cache: log the number of scanned files to trace2

Report the number of files in the working directory that were read and
their hashes verified in `refresh_index()`.

FSMonitor improves the performance of commands like `git status` by
avoiding scanning the disk for changed files.  Let's measure this.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>