merge-ort: propagate callback errors from traverse_trees_wrapper()

traverse_trees_wrapper() saves entries from a first pass through traverse_trees() and then replays them through the real callback (collect_merge_info_callback). However, the replay loop silently discards the callback return value. This is not a deferred error; it is an ignored error. Today the only originator of a negative return in this entire call graph is traverse_trees()'s "exceeded maximum allowed tree depth" check; everything else (collect_merge_info_callback, traverse_trees_wrapper, the inner traverse_trees recursion) only relays that. So in current Git, the visible effect of dropping the replay callback's return value is narrow but bad: a tree nested past core.maxTreeDepth has its -1 swallowed, the subtree below the limit is silently pruned, and the merge completes as if that were the correct result. A later patch in this series will teach collect_merge_info_callback() to return -1 on an additional path -- detecting duplicate entries in malformed trees -- which is similarly handled today by just ignoring the problem (resulting in mostly a "last one wins" rule, though the non-last entry can mutate various state flags). Capture the return value, stop the loop on negative returns, and propagate the error to the caller. The callback returns a positive mask value on success, so normalize non-negative returns to 0 for the caller. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>