add: speed up cmd_add() by utilizing read_cache_preload()

During an "add", a call is made to run_diff_files() which calls
check_removed() for each index-entry.  The preload_index() code
distributes some of the costs across multiple threads.

Because the files checked are restricted to pathspec, adding
individual files makes no measurable impact but on a Windows repo
with ~200K files, 'git add .' drops from 6.3 seconds to 3.3 seconds
for a 47% savings.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
