Here is alternative diff. Rather than checking after every message
received, we can just allocate the got_object_id once we receive a
GOT_IMSG_COMMIT_TRAVERSAL_DONE. This is here in case checking to see if
changed_commit_id is not null after each imsg is ugly.`

diff /home/kyle/src/got
commit - 39910b637a9a53cc48b0c63766da691dec0af593
path + /home/kyle/src/got
blob - 857c09bbb3a5d9425176ed93727a912b2242aac6
file + lib/privsep.c
--- lib/privsep.c
+++ lib/privsep.c
@@ -2730,8 +2730,7 @@ got_privsep_recv_traversed_commits(struct got_commit_o
 
 				/* The last commit may contain a change. */
 				if (i == icommits->ncommits - 1) {
-					*changed_commit_id =
-					    got_object_id_dup(&qid->id);
+					*changed_commit_id = &qid->id;
 					if (*changed_commit_id == NULL) {
 						err = got_error_from_errno(
 						    "got_object_id_dup");
@@ -2749,6 +2748,7 @@ got_privsep_recv_traversed_commits(struct got_commit_o
 			    datalen, ibuf);
 			break;
 		case GOT_IMSG_COMMIT_TRAVERSAL_DONE:
+			*changed_commit_id = got_object_id_dup(*changed_commit_id);
 			done = 1;
 			break;
 		default: