maybe add a --verify_existing_comments flag.

[GoogleCodeExporter]

What steps will reproduce the problem?
1. run the github_issue_converter.py migration tool
2. in the middle of transferring comments, abort the script. In my case it was 
aborted due to the abuse filter kicking in.
3. run the github_issue_converter.py migration tool once more

What is the expected output? What do you see instead?
Expected that the tool picked up where it left of by continuing the migration 
of not yet transferred comments. But instead it skips this issue completely and 
continues with the next.

What version of the product are you using? On what operating system?
Linux *** 3.13.0-43-generic #72~precise1-Ubuntu SMP Tue Dec 9 12:14:18 UTC 2014 
x86_64 x86_64 x86_64 GNU/Linux

Original issue reported on code.google.com by [email protected] on 13 Mar 2015 at 12:10