Extract repository command construction from Repositories

Summary:
Ref T4292. Ref T10366. Depends on D15751. Today, generating repository commands is purely a function of the repository, so they use protocols and credentials based on the repository configuration.

For example, a repository with an SSH "remote URI" always generate SSH "remote commands".

This needs to change in the future:

• After T10366, repositories won't necessarily just have one type of remote URI. They can only have one at a time still, but the repository itself won't change based on which one is currently active.
• For T4292, I need to generate intracluster commands, regardless of repository configuration. These will have different protocols and credentials.

Prepare for these cases by separating out command construction, so they'll be able to generate commands in a more flexible way.

Test Plan:

• Added unit tests.
• Browsed diffusion.
• Ran bin/phd debug pull to pull a bunch of repos.
• Ran daemons.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4292, T10366

Differential Revision: https://secure.phabricator.com/D15752