Copy ^/head/.arcconfig to ^/vendor/.arcconfig

[ngie]

Summary

Having a copy of .arcconfig in ^/vendor will allow developers to submit
reviews more easily using arc diff. Reviewing commits to ^/vendor has
not been standard operating procedure, as far as I can tell, but since I am
mentored and getting back up to speed, I needed to submit the review via
Phabricator.

Test Plan

I was able to submit the review via arc diff without specifying any of the
variables needed to do so.

[ngie]

@emaste, @jtl: ping?

[jtl]

LGTM

[jtl]

By the way, in the commit message, please make it clear that this simple "allows" you to more easily use arc if you want your changes to be reviewed. It doesn't require it in any way, shape, or form. And, we're not changing any expectations regarding when someone will get review.

[ngie]

In D18621#401774, @jtl wrote:

By the way, in the commit message, please make it clear that this simple "allows" you to more easily use arc if you want your changes to be reviewed. It doesn't require it in any way, shape, or form. And, we're not changing any expectations regarding when someone will get review.

Agreed :).

I submitted the following commit message to clarify the points you brought up (thank you!):

Copy ^/head/.arcconfig to ^/vendor/.arcconfig

Having a copy of .arcconfig in ^/vendor will allow developers to submit
reviews more easily and in a manner consistent to submitting diffs (out of the
box) via `arc diff` in `^/head`. This change eliminates the need for setting
these values [globally] via `arc set-config`.

Approved by:	jtl (mentor)
Differential Revision:	https://reviews.freebsd.org/D18621