grep -O: allow optional argument specifying the pager (or editor)

grep -O: allow optional argument specifying the pager (or editor)

Suppose you want to edit all files that contain a specific search term.
Of course, you can do something totally trivial such as

	git grep -z -e <term> | xargs -0r vi +/<term>

but maybe you are happy that the same will be achieved by

	git grep -Ovi <term>

now.

[jn: rebased and added tests]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Paolo Bonzini <bonzini@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>