Prevent comint prompt being pushed into comint-input-ring
Prevent comint prompt being pushed into comint-input-ring

I'm running emacs 25 and was surprised to find sometimes that the
"lfe> " input prompt of inferior-lfe appears in the REPL history. To
reproduce (assuming M-n is bound to comint-next-input):

M-x run-lfe
'foo ; to produce an initial history entry
M-n
M-n

The first comint-next-input renders 'foo, the second renders lfe> ,
and it's read-only and can't be deleted, I tend to hit enter and get
back:

** exception error: symbol lfe> is unbound

Looking into fixing this, I found inf-clojure had the same issue some
time back:

https://github.com/oyvindstegard/inf-clojure/commit/ac08fdd2b58beb373567b29c189a1d0450575a42

lfe/lfe,lfe/lfe