While that kind of magic would be quite helpful in most cases, I could imagine posts where either the author did not intend to link to a specific line (but got the link from somewhere else, or clicked on the line), but actually the file (or that line in the file), in the latest version.
Maybe a hint in a popover:
Would you like to use a permalink instead? [learn more]
Where the learn more button expands the popup and explains a bit about permalinks.