Introduce methods for accessing dynamic arguments

Change the way arguments for diff context, commit order and ignore space
are handled. Instead of using a static buffer they are now generated on
the fly when constructing the argument arrays.
Add method accessor for --show-notes argument
Always handle options linked to git arguments

This ensures that options that can also be set via the command line are
always handled by set_option to void those options are reported as
unknown.

Fixes #250