Don't do the "check which key is bigger" dance with the master context
If we receive a COMMIT message after sending our own COMMIT, it used to be the case that this was a simultaneous attempt to start OTR from each side. Now it's much more likely that one side sent its COMMIT some time in the past, but it stuck around in the master context, just in case some other logged in instance of the buddy is still going to respond with a DHKEY message. So if we now receive a COMMIT *to* the master context, ignore the old COMMIT we sent, and always use the received one to start a new SIGMA protocol.
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