Merge branch 'kh/branch-ref-syntax-advice'

Merge branch 'kh/branch-ref-syntax-advice'

When git refuses to create a branch because the proposed branch
name is not a valid refname, an advice message is given to refer
the user to exact naming rules.

* kh/branch-ref-syntax-advice:
  branch: advise about ref syntax rules
  advice: use double quotes for regular quoting
  advice: use backticks for verbatim
  advice: make all entries stylistically consistent
  t3200: improve test style