Allow passing pipes to diff --no-index + bugfix

Hello,

This set of patches adds the ability to generate diffs directly from shell process substitution using the <(...) syntax. This is extremely useful to generate diffs with files streamed directly form remote systems or when it may be useful to filter them on the fly to generate the diffs.

For example:

  $ git diff --stat \
    <(sed -r 's/^\S+\s//' /boot/System.map-4.19.0-8-amd64|sort) \
	<(sed -r 's/^\S+\s//' /boot/System.map-4.19.0-9-amd64|sort)

   /dev/fd/{63 => 62} | 9500 ++++++++++++++++++++++----------------------
   1 file changed, 4789 insertions(+), 4711 deletions(-)

Along with it a small fix in --stat and --numstat that affected one one git range-diff test, where added/removed lines stts were missing (needed for difffing the pipes too)

Regards,

Thomas Guyot-Sionnest