allow linking in diffs

path.Match does not work as expected; but our ACLs currently do not make
use of these anyway. glob/wildcard matching can be implemented in the
future.

Also make the repo action button font size text-sm.

drive-by fixes:
- make the edit icon for repo-descriptions more compact
- do not double-pad rendered blob contents

Turns out we were accidentally using the destination knot as the
sourceUrl, and git was understandably annoyed. This also improves the
error checking when fetching the existingRepo from the database to
verify the forkName won't clash.