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

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>