In short, %c should just give you the next rune, period.
Apparently this is the design. I use the term loosely.

Fixes #12275

Change-Id: I6f30bed442c0e88eac2244d465c7d151b29cf393
Reviewed-on: https://go-review.googlesource.com/13821Reviewed-by: Andrew Gerrand <adg@golang.org>