CLI: Create ast_cli_completion_vector.

This is a rewrite of ast_cli_completion_matches using a vector to build
the list.  The original function calls the vector version, NULL
terminates the vector and extracts the elements array.

One change in behavior the results are now sorted and deduplicated. This
will solve bugs where some duplicate checking was done before the list
was sorted.

Change-Id: Iede20c5b4d965fa5ec71fda136ce9425eeb69519