Merge branch 'jk/robustify-parse-commit'

* jk/robustify-parse-commit:
  checkout: do not die when leaving broken detached HEAD
  use parse_commit_or_die instead of custom message
  use parse_commit_or_die instead of segfaulting
  assume parse_commit checks for NULL commit
  assume parse_commit checks commit->object.parsed
  log_tree_diff: die when we fail to parse a commit

builtin/show-branch.c

@@ -227,8 +227,7 @@ static void join_revs(struct commit_list **list_p,
 			parents = parents->next;
 			if ((this_flag & flags) == flags)
 				continue;
-			if (!p->object.parsed)
-				parse_commit(p);
+			parse_commit(p);
 			if (mark_seen(p, seen_p) && !still_interesting)
 				extra--;
 			p->object.flags |= flags;