Do not default to --no-index when given two directories.
Do not default to --no-index when given two directories.

git-diff -- a/ b/ always defaulted to --no-index, primarily
because the function is_in_index() was implemented quite
incorrectly.

Noticed by Patrick Maaß and Simon Schubert independently,
initial patch was provided by Patrick but I fixed it
differently.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git