p7519: add trace logging during perf test

Add optional trace logging to allow us to better compare performance of
various fsmonitor providers and compare results with non-fsmonitor runs.

Currently, this includes Trace2 logging, but may be extended to include
other trace targets, such as GIT_TRACE_FSMONITOR if desired.

Using this logging helped me explain an odd behavior on MacOS where the
kernel was dropping events and causing the hook to Watchman to timeout.

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

t/perf/Makefile

@@ -7,10 +7,10 @@ perf: pre-clean
 	./run
 
 pre-clean:
-	rm -rf test-results
+	rm -rf test-results test-trace
 
 clean:
-	rm -rf build "trash directory".* test-results
+	rm -rf build "trash directory".* test-results test-trace
 
 test-lint:
 	$(MAKE) -C .. test-lint