[PATCH] manifest: check 'if x is None' instead of 'if not x'

[Yuya Nishihara]

On Sun, 26 Feb 2017 10:17:31 -0800, Durham Goode wrote:
> # HG changeset patch
> # User Durham Goode <durham at fb.com>
> # Date 1488133007 28800
> #      Sun Feb 26 10:16:47 2017 -0800
> # Node ID a21671dd2578fea1cb58011c392f6dec16f29dc7
> # Parent  88203f26ea57627cabd7cf9c4f7843661d6c43ae
> manifest: check 'if x is None' instead of 'if not x'
> 
> The old code here would end up executing __len__ on a tree manifest to determine
> if 'not _data' was true or not. This was very expensive on large repos. Since
> this function just cares about memoization, we can just check 'if _data is None'
> instead and save a bunch of time.

Good catch. Queued, thanks.