obshistory: omit duplicate notes of obsolescence operations

When folding changesets, it's possible to provide a --note flag to e.g. fold or prune, which will store a note for that operation. But since there would be more than one marker for the fold operation and the provided note would be stored in all of them, _markersnotes() previously would return the same text message many times. We don't really want to see the same thing over and over, so let's make notes a set().

This will also make unrelated operations with the same note show the message only once, which is probably an improvement too.