This fixes a problem exposed by the recent change to tags
(a2839de936). That recorded tag links as
absolute by including a leading slash in the link. The same could also be
done with an absolute wikilink.
In either case, link() would not match such links, unless the leading slash
was included in the link to match. But that's not right, because pagespecs
match absolute by default. So strip the leading slash.
Note that to keep any existing `link(/foo)` pagespecs working after this
change, the leading slash is removed from there, too.
7f80e52543