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
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