Help: prefer commit hashes in commit references
The logic is that the describe output is readily available using `git tag --contains` locally. In addition, for a hypothetical commit which landed in both v3.9.4 and v3.10.1, there is no "better" tag to refer to. since v3.10.0's relation to such a commit is unclear either way. Also mention that a `Fixes` trailer is preferred if the mention is just to indicate a commit which introduced an error rather than writing a complete sentence about it.