Add 'dulwich diff' command.

bin/dulwich

@@ -34,6 +34,7 @@ from dulwich.client import get_transport_and_path
 from dulwich.errors import ApplyDeltaError
 from dulwich.index import Index
 from dulwich.pack import Pack, sha_to_hex
+from dulwich.patch import write_tree_diff
 from dulwich.repo import Repo
 from dulwich.server import update_server_info
 
@@ -98,6 +99,20 @@ def cmd_log(args):
         todo.extend([p for p in commit.parents if p not in done])
 
 
+def cmd_diff(args):
+    opts, args = getopt(args, "", [])
+
+    if args == []:
+        print "Usage: dulwich diff COMMITID"
+        sys.exit(1)
+
+    r = Repo(".")
+    commit_id = args[0]
+    commit = r[commit_id]
+    parent_commit = r[commit.parents[0]]
+    write_tree_diff(sys.stdout, r.object_store, parent_commit.tree, commit.tree)
+
+
 def cmd_dump_pack(args):
     opts, args = getopt(args, "", [])
 
@@ -203,6 +218,7 @@ commands = {
     "clone": cmd_clone,
     "archive": cmd_archive,
     "update-server-info": cmd_update_server_info,
+    "diff": cmd_diff,
     }
 
 if len(sys.argv) < 2: