HintFmt(string) invokes the HintFmt("%s", literal) constructor,
which is not what we want here. Add a constructor with a proper name
and call that.
Next step: rename all the other ones to HintFmt::literal(string).
Fixes: https://git.lix.systems/lix-project/lix/issues/178
Change-Id: If52d2eb8864ceb8663e05992e9d1fffef573d6b8