If you try to clone a repo that doesn't exist via `https`, `git` will prompt for
auth credentials and hang forever. Using `git` avoids this and fails immediately
instead, which is what we want for a missing repo.
Part of https://github.com/vector-im/riot-web/issues/9221
Currently, people with push access to the main Riot repos can push matching
branch names to Riot and the SDKs, and CI will test all the branches together.
This change allows contributors to access the same ability when submitting
several matching PRs from their fork of each repo.
Part of https://github.com/vector-im/riot-web/issues/9041
Travis Ralston