The only exception is when moving a custom patch for an entire commit to an
earlier commit; in this case the source commit becomes empty, but we want to
keep it, mainly for consistency with moving the patch to a later commit, which
behaves the same.
In all other cases where we rebase, it's confusing when empty commits are kept;
the most common example is rebasing a branch onto master, where master already
contains some of the commits of our branch. In this case we simply want to drop
these.
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