replace usage of os.popen() with util.popen()
To make this possible, I added a mode parameter to both implementations of
util.popen(), defaulting to 'r' (as it does in the Python stdlib).
% create test repository
adding a
adding b
adding c
% churn all
user3 3 ***************************************************************
user2 2 ******************************************
user1 1 *********************
% churn up to rev 1
user2 2 ***************************************************************
user1 1 *******************************
% churn with aliases
alias3 3 **************************************************************
user2 2 *****************************************
alias1 1 ********************
% churn with column specifier
user3 3 ***********************
user2 2 ***************
user1 1 *******