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

[Thomas Guyot-Sionnest <tguyot@xxxxxxxxx>]

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