Build: better support for empty default branch

[stsewd]

The basic idea is to have the value of latest in sync with the value from the default branch, which we already do. But, in case the default branch is left empty, we were falling back to master, which is just a guess of the default, so instead of guessing the value, we just don't update it if we don't know its value. When cloning the repo, we have access to the default branch, so that's when we update latest for those projects.

Note this is only relevant for projects that aren't linked to a remote repository, since for those we know the default branch. So, when we try to build the latest version of a project that doesn't have a default branch, we:

• Omit the ref from the fetch command.
• Use the default branch from the repository for the checkout (we could skip this step, but since we have a new feature that keeps the files from a previous clone, we need to do a checkout to make sure we are building from the given branch/commit).
• Update the identifier from latest to the default branch we detected from the cloned repository.

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 (now users aren't blocked at least), and changing the default branch of a repo isn't something users do that often, and this only affects projects that aren't linked to a repository, so I think we are okay with that for now.

This replaces #10927
Closes #12231

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Documentation build overview

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[stsewd]

This is introducing an extra query for each time latest is returned in a response, since we now rely on latest having the correct value of the default branch (similar to get_original_stable_version).

[humitos]

What happens if the default branch is main, we detect and save than, and then the user changes the default branch in GitHub to dev? Do we detect that change?