Only allow `arc branch` to be used under `git`
Only allow `arc branch` to be used under `git`

Summary: Explicitly declare that the `arc branch` command is only supported under `git`.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11552

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