Consider committer data, not just commit author

[smola]

A commit has a commit author and committer. They may be the same or not. It seems we are currently using only author data and not committer.

[vmarkovtsev]

@irinakhismatullina please check this: what do we use to build the graph?

[irinakhismatullina]

Yes, we definitely use commit authors, not committers.

[irinakhismatullina]

I guess it depends on which of them we want to analyze. I don't know very well how big is the difference between the two in practice, but it seems that in theory we are more interested with the authors of the code. And also I'd expect that there were more bots among committers than among authors, but I'm not sure