Instead of taking advantage of a struct string_list that is
allocated with all NULs happens to be STRING_LIST_INIT_NODUP kind,
initialize them explicitly as such, to document their behaviour
better.
* jk/string-list-static-init:
use string_list initializer consistently
blame,shortlog: don't make local option variables static
interpret-trailers: don't duplicate option strings
parse_opt_string_list: stop allocating new strings
