merge-ort: capture and print ll-merge warnings in our preferred fashion

Instead of immediately printing ll-merge warnings to stderr, we save them in our output strbuf. Besides allowing us to move these warnings to a special file for --remerge-diff, this has two other benefits for regular merges done by merge-ort: * The deferral of messages ensures we can print all messages about any given path together (merge-recursive was known to sometimes intersperse messages about other paths, particularly when renames were involved). * The deferral of messages means we can avoid printing spurious conflict messages when we just end up aborting due to local user modifications in the way. (In contrast to merge-recursive.c which prematurely checks for local modifications in the way via unpack_trees() and gets the check wrong both in terms of false positives and false negatives relative to renames, merge-ort does not perform the local modifications in the way check until the checkout() step after the full merge has been computed.) Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>