Moving fb extensions to core.

[Yuya Nishihara]

On Wed, 5 Jul 2017 11:24:59 -0700, Durham Goode wrote:
> On 7/5/17 11:13 AM, Pulkit Goyal wrote:
> > copytrace: This patch speeds up the copy tracing by a file based
> > heuristics. I am not sure about the way to go with this.
> 
> I'd put this in core and behind a config flag.  Just like we have 
> aggressive merge deltas behind a config flag, this is an optimization 
> that is useful for some, is awkward to do via an extension, and could 
> end up replacing the current copy tracing logic in the future if we 
> iterate on it enough.

Sounds good, though I don't know the implementation details. Given how
expensive the current copy tracing is, fast-but-inaccurate one should be
a good option for large repos.