gn2bp: rename _is_append_arg for clarity

I think _is_list_arg is a slightly better name, so it isn't going to be
confused with _append_arg (which will be introduced shortly).

Thought about naming some more, and in general we have 3 types of args:
- flags (arg without value)
- value arg (arg with value)
- list arg (multiple args with multiple values)

Will work on cleaning up the grammar.

Test: none
Change-Id: I40491b33780c7ab38e926efea2b28f1135c40e0c
1 file changed