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.
If you ignore this error, there is high probability you will have a segault. As @carlosmn said, this indicates programmer error, you cannot recover from it.