Does not work well on massive git repos

_Just opening the issue for other people for documentation purposes; I'm unlikely to open a PR, because I will have to anyway move away from this action to a different one for unrelated reasons (I need the git commits to be signed)._

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The action does not work well on massive git repos. It effectively requires full git clone currently, and when the `checkout` action is used with `fetch-depth: 1` this action becomes very slow:

- first, if the action is not run from default branch, it will try to checkout and fetch the default branch (unqualified fetch, i.e. fetching all history);
- second, it will try to fetch/checkout the configured `branch` to see if there's already a PR, to update it.

I believe it should be possible to make this work with big repos, but I'd have to have code modified to avoid those expensive unqualified fetches, and possibly have limited functionality. For example, it should not check if `branch` exists by trying to fetch & check it out; it should rather do this via GH API; and given that it can't quickly fetch such branch and rebase (as the history can be arbitrarily diverged with default branch), it would have to always recreate the commits from scratch (as requested in #555), instead of rebasing.