dev/sg: detect completion mode and omit setup

Created by: bobheadxi

Partially reverts https://github.com/sourcegraph/sourcegraph/pull/33868 , closes https://github.com/sourcegraph/sourcegraph/issues/33873

Fixes the rev-list call in the update check, and just discard "bad revision" messages

Test plan

$ git rev-list asdfasdf..main -- ./dev/sg
fatal: bad revision 'asdfasdf..main'

and a bunch of manual testing