Fix git-fetch while on detached HEAD not to give needlessly alarming errors

When we are on a detached HEAD, there is no current branch.
There is no reason to leak the error messages to the end user
since this is a situation we expect to see.

This adds -q option to git-symbolic-ref to exit without issuing
an error message if the given name is not a symbolic ref.

By the way, with or without this patch, there currently is no
good way to tell failure modes between "git symbolic-ref HAED"
and "git symbolic-ref HEAD".  Both says "is not a symbolic ref".

We may want to do something about it.

Signed-off-by: Junio C Hamano <junkio@cox.net>