t7700: do not use "touch" unnecessarily

Some versions of touch (such as /usr/ucb/touch on Solaris)
do not know about the "-r" option. This would make sense as
a feature of test-chmtime, but fortunately this fix is even
easier.

The test does not care about the timestamp of the .keep file it
creates at all, only that it exists. For such a use case, with or
without portability issues around "-r", "touch" should not be used
in the first place.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7700-repack.sh

@@ -17,7 +17,7 @@ test_expect_success 'objects in packs marked .keep are not repacked' '
 	# The second pack will contain the excluded object
 	packsha1=$(git rev-list --objects --all | grep file2 |
 		git pack-objects pack) &&
-	touch -r pack-$packsha1.pack pack-$packsha1.keep &&
+	>pack-$packsha1.keep &&
 	objsha1=$(git verify-pack -v pack-$packsha1.idx | head -n 1 |
 		sed -e "s/^\([0-9a-f]\{40\}\).*/\1/") &&
 	mv pack-* .git/objects/pack/ &&