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

hgext/rebase.py

@@ -1791,7 +1791,7 b' def defineparents(repo, rev, destmap, st'
     # But our merge base candidates (D and E in above case) could still be
     # better than the default (ancestor(F, Z) == null). Therefore still
     # pick one (so choose p1 above).
-    if sum(1 for b in bases if b != nullrev) > 1:
+    if sum(1 for b in set(bases) if b != nullrev) > 1:
         unwanted = [None, None]  # unwanted[i]: unwanted revs if choose bases[i]
         for i, base in enumerate(bases):
             if base == nullrev: