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.
Status Job ID Name Coverage
  External
passed ghostflow-check-master