py3: fix manifestdict.fastdelta() to be compatible with memoryview
This doesn't look nice, but a straightforward way to support Python 3.
bytes(m[start:end]) is needed because a memoryview doesn't support ordering
operations. On Python 2, m[start:end] returns a bytes object even if m is
a buffer, so calling bytes() should involve no additional copy.
I'm tired of trying cleaner alternatives, including:
a. extend memoryview to be compatible with buffer type
=> memoryview is not an acceptable base type
b. wrap memoryview by buffer-like class
=> zlib complains it isn't bytes-like