This change makes an item of implicit coupling between the Mentionable
and CachedMarkdownField concerns. Mentionable needs to render the
fields it depends on to extract references; when those fields are also
cached markdown fields, it currently uses that cache to skip the render
step.

For the Commit model, we couldn't share the cache because Mentionable
was looking at safe_message, while CachedMarkdownField was looking at
`title`, `full_title`, and `description`. However, the combination of
the latter two is equivalent to `safe_message`, so we can use them
instead.

This opens up a couple of cans of worms - in particular, we now call
Commit#store_mentions!, which we've never done before. However, it all
seems to work out in the end.

Changelog: performance