Build: skip checkout step when building latest without an explicit default branch

[stsewd]

So, first I was trying to fix the whole webhook/build out of sync with latest, but there are several parts where default branch is set to work in some way, bogus way sometimes, but anyway. Instead of trying to solve the whole thing right now, I'm just doing two things:

• Instead of having the special case of latest identifier being None to skip the checkout step, we check for the default_branch option instead. If the default branch isn't set, it means the user wants to use the default clone branch, and instead of having to worry about keeping in for the checkout command to work, we just skip the checkout, since the default branch is already in use after cloning the repo.
• We still need to keep latest in sync with the version it points to, since that value is used in several other parts of the codebase. If the user has an explicit default branch set, that's easy, since we just take a look at that version and use that information to sync latest with, but if the user doesn't have a default branch set, we have no idea about the current default branch of the repository, we are relying on webhooks to keep this up to date, but if the repo is imported manually, and doesn't have an integration attached, RTD doesn't know what the default branch of the repo is. Git allows us to get the default branch of the repo, so now we are using that to keep latest in sync with the current default branch.

The second change probably makes updating the default branch from the webhook useless. We should probably invest a little more of time on re-structuring how latest works and how we keep it in sync, for now I just want to unblock users importing projects manually without an explicit default branch.

There's still the problem about latest's identifier being out of sync when the default branch changes, but that's fixed after a new build, and this isn't a new problem, as it's present in the current implementation (and now users aren't blocked at least), and changing the default branch of a repo isn't something users do that often, so I think we are okay with that for now.

Extra fixes:

• Set the type of latest to branch when updating from the default branch (since here it's actually a branch!)
• Always check for latest/stable and machine=True, since otherwise the versions are user-managed, and they behave as normal versions instead of rtd-managed ones.
• Include -- to end positional arguments in all git commands, this isn't a security issue, as all the git commands are run inside docker.

Closes #10915

[ericholscher]

This looks like a good change... this is mostly just to fix the issue where people change their default branch and get stuck (eg. master -> main)?

[ericholscher]

Suggested change

	# Update the latest version to point to the current default branch
	# if the project doesn't have a default branch set.
	# Update the latest version to point to the current VCS default branch
	# if the project doesn't have an explicit default branch set.

[ericholscher]

This logic is repeated 5 times in this PR... should it just be a method on self here, so that we don't have it defined in different ways?

[ericholscher]

Why would we not have a default_branch here?

[humitos]

Is this PR still relevant? I'm not sure to understand exactly what problem does it solves and how many users are affected by this. My feeling with this is that each time we touch the code on how we clone the versions we break someones' pattern 🙃 --so I'd be a little conservative here if it doesn't affect too many users.

[stsewd]

This is still valid. This solves the problem when people change their default branch, or when they manually import a project without having a default branch set explicitly.