feat(git-node): auto-fetch comparison branch when preparing release

[aduh95]

A few things in the release preparation script were still relying on the local clone to be up-to-date (e.g. to get the Current line (to prepare a LTS release), it relied on the release tag to be present, or when comparing branches). Given preparing a release requires internet access anyway, it seems easy enough to rely on it rather than the local clone.

Testing it from a codespace like this:

git clone https://github.com/nodejs/node.git --single-branch --branch=v20.x-staging --no-tags
cd node
../bin/git-node.js release --prepare

[targos]

I assume you tried it locally.