repo: Bump mtime any time we write a ref

External daemons like rpm-ostree want push notification any time a
change is made by an external entity.  inotify provides notification,
but a problem is there's no easy way to monitor all of the refs.

In the past, there has been discussion of opt-in recursive timestamps:
https://lkml.org/lkml/2013/4/5/307

But in today's world, let's just bump the mtime on the repo itself, as
a central inotify point.

Closes: https://github.com/GNOME/ostree/pull/111

tests/basic-test.sh

@@ -401,3 +401,16 @@ if test "$(id -u)" != "0"; then
     assert_file_has_content error-message "Permission denied"
     echo "ok unwritable repo was caught"
 fi
+
+cd ${test_tmpdir}
+rm -rf test2-checkout
+mkdir -p test2-checkout
+cd test2-checkout
+touch blah
+stat --printf="%Z\n" ${test_tmpdir}/repo > ${test_tmpdir}/timestamp-orig.txt
+$OSTREE commit -b test2 -s "Should bump the mtime"
+stat --printf="%Z\n" ${test_tmpdir}/repo > ${test_tmpdir}/timestamp-new.txt
+cd ..
+if ! cmp timestamp-{orig,new}.txt; then
+    assert_not_reached "failed to update mtime on repo"
+fi