scripts/get_maintainer.pl: update to current version

Update the script to version 0.26 (as of Linux v4.14-rc1)
Keep our "penguin_chief".
Keep our top_of_kernel_tree.

The negative forms of the command line parameters are described
when using --help.

New options are

 --git-blame-signatures => when used with --git-blame,
                           also include all commit signers
 --r => include reviewer(s) if any
 --letters => print all matching 'letter' types
              from all matching sections

File .get_maintainer.ignore can be used to specify
email addressees that shall be ignored.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>