Minor MIGRATE refactoring.
Minor MIGRATE refactoring.

Centralize cleanup of newargv in a single place.
Add more comments to help a bit following a complex function.

Related to issue #3016.

badboy/redis,Sciumo/redis,colstrom/redis,spinlock/redis,rogerchina/redis,neomantra/redis,nnog/redis,oranagra/redis,ofirluzon/redis,ton31337/redis,neomantra/redis,PKRoma/redis,charsyam/redis,antirez/redis,Sciumo/redis,yossigo/redis,YongMan/redis,charsyam/redis,neomantra/redis,ctripcorp/redis,soloestoy/redis,badboy/redis,pmem/redis,h0x91b/redis,charsyam/redis,zguangyu/redis,shreesundara/redis,PKRoma/redis,antirez/redis,GitHubMota/redis,ton31337/redis,OmarQunsul/graph-redis,GitHubMota/redis,hgl888/redis,ctripcorp/redis,pmem/redis,oranagra/redis,charsyam/redis,nnog/redis,spinlock/redis,OmarQunsul/graph-redis,rouzier/redis,arijitvt/redis,ton31337/redis,neomantra/redis,sunheehnus/redis,pmem/redis,arijitvt/redis,soloestoy/redis,ctripcorp/redis,Markgorden/redis,OmarQunsul/graph-redis,Markgorden/redis,shreesundara/redis,sunheehnus/redis,rogerchina/redis,zguangyu/redis,soveran/redis,dreamquster/redis,GitHubMota/redis,ctripcorp/redis,sunheehnus/redis,GitHubMota/redis,yossigo/redis,ton31337/redis,kensou97/redis,rouzier/redis,soloestoy/redis,kensou97/redis,sunheehnus/redis,YongMan/redis,antirez/redis,charsyam/redis,spinlock/redis,colstrom/redis,h0x91b/redis,soloestoy/redis,YongMan/redis,ofirluzon/redis,colstrom/redis,soveran/redis,dreamquster/redis,rogerchina/redis,antirez/redis,oranagra/redis,dreamquster/redis,Markgorden/redis,h0x91b/redis,YongMan/redis,soveran/redis,rouzier/redis,PKRoma/redis,dreamquster/redis,kensou97/redis,oranagra/redis,ofirluzon/redis,badboy/redis,arijitvt/redis,ofirluzon/redis,neomantra/redis,shreesundara/redis,rouzier/redis,zguangyu/redis,hgl888/redis,hgl888/redis,arijitvt/redis,colstrom/redis,yossigo/redis,h0x91b/redis,soveran/redis,hgl888/redis,PKRoma/redis,rogerchina/redis,Sciumo/redis,zguangyu/redis,badboy/redis,yossigo/redis,oranagra/redis,OmarQunsul/graph-redis,kensou97/redis,PKRoma/redis,soloestoy/redis,nnog/redis,Markgorden/redis,pmem/redis,nnog/redis,yossigo/redis,shreesundara/redis,ofirluzon/redis,spinlock/redis,Sciumo/redis