merge_trees(): ensure that the callers release output buffer

The recursive merge machinery accumulates its output in an output
buffer, to be flushed at the end of merge_recursive(). At this point,
we forgot to release the output buffer.

When calling merge_trees() (i.e. the non-recursive part of the recursive
merge) directly, the output buffer is never flushed because the caller
may be merge_recursive() which wants to flush the output itself.

For the same reason, merge_trees() cannot release the output buffer: it
may still be needed.

Forgetting to release the output buffer did not matter much when running
git-checkout, or git-merge-recursive, because we exited after the
operation anyway. Ever since cherry-pick learned to pick a commit range,
however, this memory leak had the potential of becoming a problem.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -573,6 +573,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 				exit(128);
 			ret = reset_tree(new->commit->tree, opts, 0,
 					 writeout_error);
+			strbuf_release(&o.obuf);
 			if (ret)
 				return ret;
 		}

merge-recursive.c

@@ -2078,6 +2078,8 @@ int merge_recursive(struct merge_options *o,
 		commit_list_insert(h2, &(*result)->parents->next);
 	}
 	flush_output(o);
+	if (!o->call_depth && o->buffer_output < 2)
+		strbuf_release(&o->obuf);
 	if (show(o, 2))
 		diff_warn_rename_limit("merge.renamelimit",
 				       o->needed_rename_limit, 0);

sequencer.c

@@ -293,6 +293,7 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 	clean = merge_trees(&o,
 			    head_tree,
 			    next_tree, base_tree, &result);
+	strbuf_release(&o.obuf);
 	if (clean < 0)
 		return clean;