[PATCH 1 of 2 V3] commitextras: move fb extension to core which add extras to a commit

[Yuya Nishihara]

On Sat, 15 Jul 2017 01:31:23 +0530, Pulkit Goyal wrote:
> # HG changeset patch
> # User Pulkit Goyal <7895pulkit at gmail.com>
> # Date 1499799225 -19800
> #      Wed Jul 12 00:23:45 2017 +0530
> # Node ID e51d188da49636884ae6c0df94f501e84436b857
> # Parent  80e1331a7fe970f3e56fde9044949d72d3afdf30
> # EXP-Topic fbext
> commitextras: move fb extension to core which add extras to a commit

I think it's okay to start this as an extension, but I don't think this
extension should be advertised. So +1 for queueing this and marking it as
EXPERIMENTAL or ADVANCED as follow-up.

I'm also fine with adding a hidden config knob to unblock this feature.