> * should we worry about case-insensitive matching?

hgweb seems to accept only lowercase cs ids, so the regex can be limited
to [0-9a-f].  OTOH cs ids are lowercase everywhere so someone would have
to intentionally convert it to uppercase to break the link.

> * links to repository tags

I would do this only if prefixed by 'hg:'.  hgweb accepts tags already,
so you don't have to anything special to handle this (just have a regex
that accepts the tags).

> * links to repository revision numbers

Revision numbers (the sequential ones) shouldn't be used IMHO.

> * links to specific files/lines

I find this handy, so you could include it too.