Allow absolute paths as arguments for executable()

Thanks to filterfalse.
Document that 'incsearch' affects local filter

Thanks to filterfalse.
Repeat last search for empty pattern on search

Previous behaviour was strange, inconsistent with Vim and hard to
predict.
Skip nonexistent directories on Ctrl-O/Ctrl-I

This is not only convenient (at least for me), but also more consistent
with content of the :history menu for a view.
Add =pattern special form of user-defined command

Sets local filter to the given value (alike "/pattern" and "filter value").

Thanks to filterfalse.