git_revert test: reinstate ikiwiki.setup, and make it work uninstalled
Previously it was relying on running with an installed ikiwiki
and being able to copy in recentchanges.mdwn and wikiicons/ from the
underlay in /usr. The underlay in ./underlays/basewiki can't be used
(yet) because ikiwiki doesn't allow following symlinks, even from
underlays.
I'd like to make ikiwiki follow symlinks whose destinations can be
verified to be safe (for example making it willing to expose
/usr/share/javascript to the web, but not /etc/passwd), at least from
underlays, but this is security-sensitive so I'm not going to rush
into it.