Diviner: mention how to flag Arcanist and Phorge as "safe" git repos

Summary:
This tip is useful if you are affected by the following error in git or similar:

fatal: detected dubious ownership in repository at '/var/www/phorge'

Just follow the new tip from Diviner to fix at system level.

Ref T15282

Test Plan:

• see if you like the documentation tip
• try to execute the command, and verify that your computer does not explode
• eventually visit the /config/ page with and without that change (with and without a /etc/gitconfig)

Reviewers: O1 Blessed Committers, avivey, Cigaryno

Reviewed By: O1 Blessed Committers, avivey, Cigaryno

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15282

Differential Revision: https://we.phorge.it/D25236