rev-parse: make "whatchanged -- git-fetch-script" work again.

The latest update to avoid misspelled revs interfered when we
were not interested in parsing non flags or arguments not meant
for rev-list.  This makes these two forms work again:

git whatchanged -- git-fetch-script

We could enable "!def" in the part this change touches to make
the above work without '--', but then it would cause misspelled
v2.6.14..v2.6.16 to be given to diff-tree and defeats the whole
point of the previous fix.

Signed-off-by: Junio C Hamano <junkio@cox.net>