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.
#!/bin/sh
mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m test -d "1000000 0"
hg history
hg manifest --debug
hg cat a
hg verify