merge: different msg if nothing was merged

OpenKeychain/src/main/res/values/strings.xml

@@ -636,6 +636,7 @@
     <string name="msg_mg_heterogeneous">Tried to consolidate heterogeneous keyrings</string>
     <string name="msg_mg_new_subkey">Adding new subkey %s</string>
     <string name="msg_mg_found_new">Found %s new certificates in keyring</string>
+    <string name="msg_mg_unchanged">No new certificates</string>
 
     <!-- createSecretKeyRing -->
     <string name="msg_cr">Generating new master key</string>