404 when selecting a bookmark to compare against branch in branch detail (BB-9786)

Looking at the mutable history project for mercurial I find that if I try to compare to a bookmark I'm brought to the dead link page. For example if I start off at:

https://bitbucket.org/marmoute/mutable-history/branch/default?dest=stable

and then change the destination to '@', I'm brought to:

https://bitbucket.org/marmoute/mutable-history/branch/default?dest=%40