rewriteutil: handle dropped commits when updating description hashes In looking to leverage this with the absorb extension, the old -> new mapping there allows the new value to be None. We could filter that out and not pass it to this method, but it seems worth a message to the user. (I wonder if these should be an info or warning, because it's unlikely people are using `-v` regularly.) Differential Revision: https://phab.mercurial-scm.org/D9181