cvsimport: standarize open() calls to external git tools

cvsimport: standarize open() calls to external git tools

Standardize calls to open() where external git tools are used as
part of a pipeline.  Instead of open(X, "git foo ... |)", use
open(X, "-|", qw(git foo ...)).  All calls are made without the
use of an 'sh -c' process to split the arguments.

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>