Currently the --no-index option is parsed in diff_no_index(). Move the
detection if a no-index diff should be executed to builtin/diff.c, where
we can use it for executing diff_no_index() conditionally. This will
also allow us to execute other operations conditionally, which will be
done in the next patch.
There are no functional changes.
Helped-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>