[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.

changeset:   0:9426b370c206
tag:         tip
user:        My Name <myname@example.com>
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     commit-1