merged net-update fixes from apt--mvo

cmdline/apt-key

@@ -58,8 +58,19 @@ net_update() {
     if [ ! -d /var/lib/apt/keyrings ]; then
 	mkdir -p /var/lib/apt/keyrings
     fi
+    keyring=/var/lib/apt/keyrings/$(basename $ARCHIVE_KEYRING)
+    old_mtime=0
+    if [ -e $keyring ]; then
+	old_mtime=$(stat -c %Y $keyring)
+    fi
     (cd  /var/lib/apt/keyrings; wget -q -N $ARCHIVE_KEYRING_URI)
-    add_keys_with_verify_against_master_keyring /var/lib/apt/keyrings/$(basename $ARCHIVE_KEYRING) $MASTER_KEYRING
+    if [ ! -e $keyring ]; then
+	return
+    fi
+    new_mtime=$(stat -c %Y $keyring)
+    if [ $new_mtime -ne $old_mtime ]; then
+	add_keys_with_verify_against_master_keyring $keyring $MASTER_KEYRING
+    fi
 }
 
 update() {