Add sg doctor check for git version.
Created by: varungandhi-src
It's easy to run into a mysterious gitserver failure when running sg start
because of an outdated Git. Example: https://sourcegraph.slack.com/archives/C07KZF47K/p1642491532171300
On macOS, Xcode CommandLineTools include Git 2.30.1 whereas we need 2.34.1 or newer.