Add trace2_region_enter() and trace2_region_leave() calls around the
various phases of a status scan. This gives elapsed time for each
phase in the GIT_TR2_PERF and GIT_TR2_EVENT trace target.
Also, these Trace2 calls now use s->repo rather than the_repository.
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>