t3419-*.sh: Fix arithmetic expansion syntax error

Some shells, for example dash versions older than 0.5.4, need to
spell a variable reference as '$N' rather than 'N' in an arithmetic
expansion. In order to avoid the syntax error, we change the
offending variable reference from 'i' to '$i' in function scramble.

There is nothing bash specific to this test script (and we shouldn't
have any bash dependent test).  Fix its shebang line.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3419-rebase-patch-id.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 test_description='git rebase - test patch id computation'
 
@@ -27,7 +27,7 @@ scramble()
 		then
 			echo "$x"
 		fi
-		i=$(((i+1) % 10))
+		i=$((($i+1) % 10))
 	done < "$1" > "$1.new"
 	mv -f "$1.new" "$1"
 }