Teach "git diff" to honour --[no-]ext-diff

Teach "git diff" to honour --[no-]ext-diff

The original intention of 72909be (Add diff-option --ext-diff, 2007-06-30)
was to optionally allow the use of external diff viewer in "git log"
family (while keeping them disabled by default).  It exposed the "allow
external diff" bit to the UI, but forgot to adjust the "git diff" codepath
that was set up to always allow use of the external diff viewer.

Noticed by Nazri Ramliy; tests by René Scharfe squashed in.

Signed-off-by: Junio C Hamano <gitster@pobox.com>