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
hg init a
cd a
hg log
hg grep wah
hg manifest
hg verify
ls .hg
ls .hg/store
cd ..
hg clone a b
cd b
hg verify
ls .hg
ls .hg/store