Rebasing can try to stash huge files

When `pull rebase` is used, all the changes are stashed in the user's repository, so the rebasing can be done in a clean state. For that `--all` is used when stashing, which stashes also ignored files, because when we checkout the pull request branch, we don't know if those files won't be ignored any more so they can clash with the local files.

The thing is, if the user has very big untracked or ignored files, git stash will have to stash huge amounts of data, which is far from ideal.

It might be good to consider doing a complete new checkout in a temporary directory to do the rebase, this way we don't have to mess with the user's working copy **at all**.