branch: 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.  Use $curr_branch_short in the function
error_on_no_merge_candidates instead of removing the prefix from
$curr_branch directly.

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>