git-debrebase: Fix error message for wrong use of -i

git-debrebase: Fix error message for wrong use of -i

Getopt::Long with "i:s" parses "-i something" as "-i" with a value of
"something".  Maybe I didn't know this when I wrote this code,
and thought the check for $val would only reject "-isomething".

But "git-rebrebase ... -i something" is wrong because it would mean
"git-rebase -i something" and we do not permit the user to specify
their own base.  So it is right to bail out in this case.

However, the message was wrong, since it refers to cuddling, and it
can be caused by a non-cuddled non-option argument.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>