In Git, fall back across versions more cleanly when trying to get the URI for a remote

Summary: Fixes T13481. When identifying the URI for a remote, fall back from "git remote get-url" to "git ls-remote --get-url" to "git config remote.<name>.url" based on command output and version tests.

Test Plan: Ran arc land --hold, rigged the subcommands to fail to try all three fallbacks, ran arc land --hold --remote asdfasdf to get an explicit failure.

Maniphest Tasks: T13481

Differential Revision: https://secure.phabricator.com/D20950