pull: use $curr_branch_short more

One of the first things git-pull.sh does is setting $curr_branch to
the target of HEAD and $curr_branch_short to the same but with the
leading "refs/heads/" removed.  Simplify the code by using
$curr_branch_short instead of setting $curr_branch to the same
shortened value.

The only other use of $curr_branch in that function doesn't have to
be replaced with $curr_branch_short because it just checks if the
string is empty.  That property is the same with or without the prefix
unless HEAD points to "refs/heads/" alone, which is invalid.

Noticed-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -166,9 +166,8 @@ error_on_no_merge_candidates () {
 		op_prep=with
 	fi
 
-	curr_branch=${curr_branch#refs/heads/}
-	upstream=$(git config "branch.$curr_branch.merge")
-	remote=$(git config "branch.$curr_branch.remote")
+	upstream=$(git config "branch.$curr_branch_short.merge")
+	remote=$(git config "branch.$curr_branch_short.remote")
 
 	if [ $# -gt 1 ]; then
 		if [ "$rebase" = true ]; then