Fixed issue #1886: Fetch&Rebase should allow automatic branch update if FF

Just a note:

I ever think that looking up whether the arbitrary URL matches one of remote' URL, then changing the ```url``` to matched remote automatically.

The difference is: (when URL = one remote's URL)
 * If ```url``` is a URL, only FETCH_HEAD is updated. So, only the FETCH_HEAD can be used for rebase and FF.
 * if ```url``` is a remote, then the remote tracking branch is also updated and remote tracking branch is FF.

But, might be not a good idea.
Because git does not do that by default.