autoindent: fix indentation when "keep extra spaces" is enabled

This changes the way that the "keep extra spaces" option affects
keepIndent, which is called when an indentation script returns -1.
Instead of adding all the indentation of the current line to that of the
previous line, we only add the extra spaces that are not a multiple of
the indentation width. This is more consistent with how the option is
used everywhere else.

In addition, the "keep extra spaces" option is temporarily disabled when
re-indenting a block of code, which happens when pasting code or through
Selection->Format Indentation. Since we're supposed to re-indent the
code block from scratch, it makes no sense to keep extra spaces around.