t5583: fix shebang line

The shebang was missing the leading `/` character, resulting in:

    $ ./t5583-push-branches.sh
    bash: ./t5583-push-branches.sh: cannot execute: required file not found

Add the missing character so the test can run.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5583-push-branches.sh

@@ -1,4 +1,4 @@
-#!bin/sh
+#!/bin/sh
 
 test_description='check the consisitency of behavior of --all and --branches'