git-rebase: report checkout failure

When detaching the HEAD to the base commit, the "git checkout" command
could fail if, for example, upstream contains a file that would overrwrite
a local, untracked file.  Unconditionally discarding the standard error
stream was done to squelch the progress and notices back when checkout
did not have -q option, but there is no reason to keep doing it anymore.

Noticed by Robert Shearman.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -376,9 +376,7 @@ fi
 
 # Detach HEAD and reset the tree
 echo "First, rewinding head to replay your work on top of it..."
-git checkout "$onto^0" >/dev/null 2>&1 ||
+git checkout -q "$onto^0" || die "could not detach HEAD"
-	die "could not detach HEAD"
-# git reset --hard "$onto^0"
 
 # If the $onto is a proper descendant of the tip of the branch, then
 # we just fast forwarded.