tests: Fix race condition in t7006-pager

tests: Fix race condition in t7006-pager

Pagers that do not consume their input are dangerous: for example,

 $ GIT_PAGER=: git log
 $ echo $?
 141
 $

The only reason these tests were able to work before was that
'git log' would write to the pipe (and not fill it) before the
pager had time to terminate and close the pipe.

Fix it by using a program that consumes its input, namely wc (as
suggested by Johannes).

Reported-by: Johannes Sixt <j.sixt@viscovery.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>