diff: turn --ita-invisible-in-index on by default

Due to the implementation detail of intent-to-add entries, the current
"git diff" (i.e. no treeish or --cached argument) would show the
changes in the i-t-a file, but it does not mark the file as new, while
"diff --cached" would mark the file as new while showing its content
as empty.

     $ git diff                     | $ diff --cached
    --------------------------------|-------------------------------
     diff --git a/new b/new         | diff --git a/new b/new
     index e69de29..5ad28e2 100644  | new file mode 100644
     --- a/new                      | index 0000000..e69de29
     +++ b/new                      |
     @@ -0,0 +1 @@                  |
     +haha                          |

One evidence of the current output being wrong is that, the output
from "git diff" (with ita entries) cannot be applied because it
assumes empty files exist before applying.

Turning on --ita-invisible-in-index [1] [2] would fix this. The result
is "new file" line moving from "git diff --cached" to "git diff".

     $ git diff                     | $ diff --cached
    --------------------------------|-------------------------------
     diff --git a/new b/new         |
     new file mode 100644           |
     index 0000000..5ad28e2         |
     --- /dev/null                  |
     +++ b/new                      |
     @@ -0,0 +1 @@                  |
     +haha                          |

This option is on by default in git-status [1] but we need more fixup
in rename detection code [3]. Luckily we don't need to do anything
else for the rename detection code in diff.c (wt-status.c uses a
customized one).

[1] 425a28e0a4 (diff-lib: allow ita entries treated as "not yet exist
    in index" - 2016-10-24)
[2] b42b451919 (diff: add --ita-[in]visible-in-index - 2016-10-24)
[3] bc3dca07f4 (Merge branch 'nd/ita-wt-renames-in-status' - 2018-01-23)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff.c

@@ -352,6 +352,13 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 	rev.diffopt.flags.allow_external = 1;
 	rev.diffopt.flags.allow_textconv = 1;
 
+	/*
+	 * Default to intent-to-add entries invisible in the
+	 * index. This makes them show up as new files in diff-files
+	 * and not at all in diff-cached.
+	 */
+	rev.diffopt.ita_invisible_in_index = 1;
+
 	if (nongit)
 		die(_("Not a git repository"));
 	argc = setup_revisions(argc, argv, &rev, NULL);