Add links for existing [xxxx] links by TracMercurial

[Marcus Lindblom]

Can this be done easily? I just added the plugin (works very well 8-D) but we have older repo-links in commits that use simpler link syntax, and having them to point to the right hg-repo would be nice. (Now they either point to the main repo that TracMercurial tracks, or nowhere)

Or, if there was a good way to modify the ​trac-mercurial-hook to match the repo-paths to the repo-ids in trac.ini, that would be a way to fix it from now on.