Fix newline substitution.

Problem  : Command `s/\n//` is being translated into a call to do_join
           with a count of 1. But do_join asserts its precondition count
           >= 2, which is causing the program to abort.
Note     : This in fact revealed bigger problems: generated join command
           line count, as well as reported substitutions/lines were
           wrong in several cases, since patch 7.4.232.
           See:
           * http://markmail.org/message/vo7ruair5raccawp
           * https://code.google.com/p/vim/issues/detail?id=287
Solution : - Don't generate join command for single-line-range case.
           - Make generated join command include:
             * lines in range + 1, when range doesn't end at last line.
             * lines in range, when range ends at last line.
           - Make reported substitutions/lines always be
             number-of-lines-joined - 1.