Use and extract functions for completing scope arguments

Not really attached to this but it enables the next commit to use
menu() for completing scopes.

This refactoring is possible because we always have

	params[token_to_complete].length() == pos_in_token

---

Instead of three separate functions, I originally tried to add
template arguments to complete_scope().  That worked fine with g++
12.1 but clang 14.0 complained when wrapping a menu() around a
complete_scope() that relied on defaulted template arguments:

	commands.cc:1087:20: error: no matching function for call to 'menu'
	    make_completer(menu(complete_scope), menu(complete_hooks), complete_nothing,
	                   ^~~~
	commands.cc:116:6: note: candidate template ignored: couldn't infer template argument 'Completer'
	auto menu(Completer completer)
	     ^

0 files changed, 0 insertions, 0 deletions