Do not error out when fetching from second remote

In `git_remote_update_tips()`, it prepares `FETCH_HEAD` file. If `HEAD` 's remote is not the same as the fetching remote, it raised an error.

This pull request fixes the issue.
You don't ignore the error, you fix it. We're running through this code regardless of whether the upstream remote is the one we're fetching from. Check for that and return instead of doing it.