[churn] Ignore merge csets
Merge changesets attributed a large number of lines to people that didn't
actually develop it, but simply merged the code. By ignoring merges, we get
a more accurate picture of how much code everyone changed.
# should print nothing
# should print ? b/x
? b/x
# should print A b/x
A b/x
# should forget b/x
forgetting b/x
# should print nothing