font-lock-constant-face in 1-char length labels

[Tino Calancha]

Hi,

i have noticed that labels are fontified with
`font-lock-constant-face' _only_ if the length of the
label is > 1.
For instance,
emacs -Q
;; inserts following in *scratch* buffer:
`foo'
`f'

The former get highlighted with `font-lock-constant-face',
but the later show `default' face.

I observe the same in master branch, emacs-25 and emacs-24, so it
seems this has being always the behaviour.
Is there a good reason for this exception with 1-char labels?

Tino