Sourcetree is not always able to link file system renames automagically in Mercurial repositories... it would be extremely helpful if a user could highlight two files in a list (one "missing" and another that is "not tracked"), right click, and choose "Rename" and basically perform an "hg rename" operation retroactively (file level rename to original name, then hg rename to new target name).
At the same time, a standard rename option in the GUI would be great for when you just want to right-click on a single file, pick rename, and enter the new name (triggering a typical "hg rename" operation).