(This patch follows from 72334, but that patch is not necessary for this
one to work.)

Currently something like the following (unevaluated) form:
  `(foo bar . ,baz)
prints as
  `(foo bar \, baz)
which, while technically accurate, is not helpful.  This comes up quite
often in pcase forms where a pattern like `(,head . ,_) is extremely
common.

This patch fixes that, such that if the tail of a list is a proper list
of two elements whose car is a backquote, comma, or comma-at (so ,foo
,@foo and `foo), and print-quoted is non-nil, the printed text will be
in the first form above.

Better ways to implement this are likely possible; this was simply the
easiest and quickest one.  Feel free to change the patch as needed.