lib/upgrader: Pull with timestamp-check-from-rev

For the same reason as https://github.com/coreos/rpm-ostree/pull/2094.
What we care most about is that the new commit we pull is newer than the
one we're currently sitting on, not necessarily that it's newer than the
branch itself, which it might not be if e.g. we're trying to deploy a
commit older than the tip but still newer than the deployment (via
`--override-commit`).

src/libostree/ostree-sysroot-upgrader.c

@@ -530,8 +530,8 @@ ostree_sysroot_upgrader_pull_one_dir (OstreeSysrootUpgrader  *self,
                              g_variant_new_variant (g_variant_new_int32 (flags)));
       /* Add the timestamp check, unless disabled */
       if ((upgrader_flags & OSTREE_SYSROOT_UPGRADER_PULL_FLAGS_ALLOW_OLDER) == 0)
-        g_variant_builder_add (optbuilder, "{s@v}", "timestamp-check",
-                               g_variant_new_variant (g_variant_new_boolean (TRUE)));
+        g_variant_builder_add (optbuilder, "{s@v}", "timestamp-check-from-rev",
+                               g_variant_new_variant (g_variant_new_string (from_revision)));
 
       g_variant_builder_add (optbuilder, "{s@v}", "refs",
                              g_variant_new_variant (g_variant_new_strv ((const char *const*) refs_to_fetch, -1)));