testsuite: fix timestamp race in exclude test (#1120)

* testsuite: fix timestamp race in exclude test

The same-newness files must have equal modification times, but
separate touch calls can straddle a second boundary and make the
destination newer. This causes --update to skip it unexpectedly.

Copy the source timestamps to the destination explicitly.

* testsuite: only copy update-test mtime

---------

Co-authored-by: Zen Dodd <mail@steadytao.com>

testsuite/exclude_test.py

@@ -137,8 +137,15 @@ up1.mkdir()
 up2.mkdir()
 (up1 / 'dst-newness').touch()
 (up2 / 'src-newness').touch()
-(up1 / 'same-newness').touch()
-(up2 / 'same-newness').touch()
+same_src = up1 / 'same-newness'
+same_dst = up2 / 'same-newness'
+same_src.touch()
+same_dst.touch()
+# Give both files the same mtime; otherwise two touch calls could straddle a
+# second boundary and --update would see the destination as newer.
+src_stat = same_src.stat()
+dst_stat = same_dst.stat()
+os.utime(same_dst, ns=(dst_stat.st_atime_ns, src_stat.st_mtime_ns))
 (up1 / 'extra-src').touch()
 (up2 / 'extra-dest').touch()