vi: further fixes to undo after autoindent

vi: further fixes to undo after autoindent

Commit bb983f30e (vi: fix faulty undo after autoinsert) has a
number of problems:

- The commit message refers to 'autoinsert' when it really means
  'autoindent'.

- The indentation of undo_push_insert() was incorrect.

- Most seriously the commit only fixed the problem for cases where
  the indentation was exactly one character.  This is because undo_push()
  only allows single characters to be queued for UNDO_INS_QUEUED.

  Lifting this restriction allows the example given in the previous
  commit message (with a three character indent) to work.

function                                             old     new   delta
undo_push                                            406     435     +29
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 29/0)               Total: 29 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>