git-checkout: fix "eval" used for merge labelling.

The symbolic notation of the fork point can contain whitespaces (e.g.
"git checkout -m 'HEAD@{9 hours ago}'").  Quote strings properly
when using eval to prepare GITHEAD_$new

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

git-checkout.sh

@@ -196,7 +196,7 @@ else
 	work=`git write-tree` &&
 	git read-tree --reset -u $new || exit
 
-	eval GITHEAD_$new=${new_name:-${branch:-$new}} &&
+	eval GITHEAD_$new='${new_name:-${branch:-$new}}' &&
 	eval GITHEAD_$work=local &&
 	export GITHEAD_$new GITHEAD_$work &&
 	git merge-recursive $old -- $new $work