rebase: make rebase not crash if p1 == p2 != nullid
While this shouldn't happen normally, some historical bugs can have caused this
kind of commit to exist. Instead of crashing and having it be unobvious how to
recover, let's try to continue on. Without this, we get an error like
"ValueError: min() arg is an empty sequence"
Differential Revision: https://phab.mercurial-scm.org/D7664
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