p7519: do not rely on "xargs -d" in test

Convert the test to use a more portable method to update the mtime on a
large number of files under version control.

The Mac version of xargs does not support the "-d" option.
Likewise, the "-0" and "--null" options are not portable.

Furthermore, use `test-tool chmtime` rather than `touch` to update the
mtime to ensure that it is actually updated (especially on file systems
with only whole second resolution).

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/p7519-fsmonitor.sh

@@ -164,8 +164,18 @@ test_fsmonitor_suite() {
 		git status -uall
 	'
 
+	# Update the mtimes on upto 100k files to make status think
+	# that they are dirty.  For simplicity, omit any files with
+	# LFs (i.e. anything that ls-files thinks it needs to dquote).
+	# Then fully backslash-quote the paths to capture any
+	# whitespace so that they pass thru xargs properly.
+	#
 	test_perf_w_drop_caches "status (dirty) ($DESC)" '
-		git ls-files | head -100000 | xargs -d "\n" touch -h &&
+		git ls-files | \
+			head -100000 | \
+			grep -v \" | \
+			sed '\''s/\(.\)/\\\1/g'\'' | \
+			xargs test-tool chmtime -300 &&
 		git status
 	'