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.
This error handling does not belong here. What it used to handle was lookup in the remote-tracking branch, which is now a bunch of lines below this. If the reference isn't a branch, we should simply return 0.