convert/gnuarch: retrieve known archive names list
This will make possible in a followup patch to prevent
following history that is not reachable because an archive
is unknown to the user.
% create test repository
adding a
adding b
adding c
% churn all
user3 3 ***************************************************************
user2 2 ******************************************
user1 1 *********************
% churn up to rev 2
user2 2 ***************************************************************
user1 1 *******************************
% churn with aliases
alias3 3 **************************************************************
user2 2 *****************************************
alias1 1 ********************
% churn with column specifier
user3 3 ***********************
user2 2 ***************
user1 1 *******
% churn by hour
06 1 **********************
09 2 ********************************************
12 3 ******************************************************************