help_unknown_ref(): duplicate collected refnames

When "git merge" sees an unknown refname, we iterate through the refs to try to suggest some possible alternates. We do so with for_each_ref(), and in the callback we add some of the refnames we get to a string_list that is declared with NODUP, directly adding a pointer into the refname string our callback received. But the for_each_ref() machinery does not promise that the refname string will remain valid, and as a result we may print garbage memory. The code in question dates back to its inception in e56181060e (help: add help_unknown_ref(), 2013-05-04). But back then, the refname strings generally did remain stable, at least immediately after the for_each_ref() call. Later, in d1cf15516f (packed_ref_iterator_begin(): iterate using `mmapped_ref_iterator`, 2017-09-25), we started consistently re-using a separate buffer for packed refs. The fix is simple: duplicate the strings we intend to collect. We already call string_list_clear(), so the memory is correctly freed. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>