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.
adding a
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
% should issue warning
adding a
created new head
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
% should not issue warning
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
marked working directory as branch testing
% should not issue warning