Docdiff: use pull request base branch for comparison

[agjohnson]

Assuming we get the base branch from pull requests events on all of our provider integrations, we should be able to set the addons configuration for the docdiff base url on a pull request basis. This covers the use case of pull requests that have varying base branches and patterns like maintaining docs for multiple minor versions -- eg pull requests might base from a 1.x, 2.x, or 3.x branch.

We've discussed this in the past and there are some hurdles to implementing this, namely that the base branch might not be an active version. So at least we need to check for that and probably fall back to a version that is active. Are there other problems with this approach?

[humitos]

Assuming we get the base branch from pull requests events on all of our provider integrations

Currently, the base branch is defined manually by AddonsConfig.options_base_version.

By the way, this issue is probably a duplicate of/related to readthedocs/readthedocs.org#12232

[agjohnson]

Currently, the base branch is defined manually by AddonsConfig.options_base_version.

Yup, but this issue is specifically allowing for a varying base branch, which cannot be achieved with a addons configuration.

By the way, this issue is probably a duplicate of/related to readthedocs/readthedocs.org#12232

Yeah I can't tell, that sounds different to me. @stsewd would addressing 12232 address the use case in the description?

[stsewd]

would addressing 12232 address the use case in the description?

Yep, my proposed solution is more general, so it works with any base branch, but it also works when the branch gets out of date with its base branch.