Summary:
Pulls the logic to define changelists from p4fastimport into separate function
and re-uses that in p4seqimport
For a high-level overview of p4seqimport, please check https://our.intern.facebook.com/intern/wiki/IDI/p4seqimport/
Differential Revision: D7035674
fbshipit-source-id: 699e9148d35e437f306062f290c8ec2a857df480
