pager test: make fake pager consume all its input

Otherwise there is a race: if 'git log' finishes writing before the
pager terminates and closes the pipe, all is well, and if the pager
finishes quickly enough then 'git log' terminates with SIGPIPE.

 died of signal 13 at /build/buildd/git-1.9~rc1/t/test-terminal.perl line 33.
 not ok 6 - LESS and LV envvars are set for pagination

Noticed on Ubuntu PPA builders, where the race was lost about half the
time.  Compare v1.7.0.2~6^2 (tests: Fix race condition in t7006-pager,
2010-02-22).

Reported-by: Anders Kaseorg <andersk@MIT.EDU>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>