[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.

% init
% commit
adding a
% annotate -c
8435f90966e4: a
% annotate -d
Thu Jan 01 00:00:01 1970 +0000: a
% annotate -n
0: a
% annotate -u
nobody: a
% annotate -cdnu
nobody 0 8435f90966e4 Thu Jan 01 00:00:01 1970 +0000: a