diff-files: treat "i-t-a" files as "not-in-index"

diff-files: treat "i-t-a" files as "not-in-index"

The `diff-files' command and related commands which call the function
`cmd_diff_files()', consider the "intent-to-add" files as a part of the
index when comparing the work-tree against it. This was previously
addressed in commits [1] and [2] by turning the option
`--ita-invisible-in-index' (introduced in [3]) on by default.

For `diff-files' (and `add -p' as a consequence) to show the i-t-a
files as as new, `ita_invisible_in_index' will be enabled by default
here as well.

[1] 0231ae71d3 (diff: turn --ita-invisible-in-index on by default,
                2018-05-26)
[2] 425a28e0a4 (diff-lib: allow ita entries treated as "not yet exist
                in index", 2016-10-24)
[3] b42b451919 (diff: add --ita-[in]visible-in-index, 2016-10-24)

Signed-off-by: Srinidhi Kaushik <shrinidhi.kaushik@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>