janitor: use NULL and not 0 for pointers.

Brought to you thanks to coccinelle:

    ---8<----
    @@
    expression *E;
    @@
    (
      E ==
    - 0
    + NULL
    |
      E !=
    - 0
    + NULL
    |
      E =
    - 0
    + NULL
    )

    @@
    identifier f;
    type T;
    @@
    T *f(...) {
    <...
    - return 0;
    + return NULL;
    ...>
    }
    --->8----

There are a lot more hits in compat/nedmallox and compat/regex but these
are borrowed code we rather do not want to maintain our own forks for,
and this patch refrains from touching them.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -893,7 +893,7 @@ static struct column *find_new_column_by_commit(struct git_graph *graph,
 		if (graph->new_columns[i].commit == commit)
 			return &graph->new_columns[i];
 	}
-	return 0;
+	return NULL;
 }
 
 static void graph_output_post_merge_line(struct git_graph *graph, struct strbuf *sb)