string_list_add_refs_by_glob(): add a comment about memory management

Since string_list_add_one_ref() adds refname to the string list, but
the lifetime of refname is limited, it is important that the
string_list passed to string_list_add_one_ref() has strdup_strings
set.  Document this fact.

All current callers do the right thing.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>