help_unknown_ref(): duplicate collected refnames

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>