pull: plug minor memory leak after using is_descendant_of()
cmd_pull() builds a commit_list to pass a single potential ancestor to
is_descendant_of(). The latter leaves the list intact. Release the
allocated memory after the call.
Leaking in cmd_*() isn't a big deal, but sets a bad example for other
users of is_descendant_of().
Signed-off-by: René Scharfe <l.s.r@web.de> Acked-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>