contrib: completion: fix 'eread()' namespace

Otherwise it might collide with a function of the same name in the
user's environment.

Suggested-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>