diff-lib.c: adjust position of i-t-a entries in diff

diff-lib.c: adjust position of i-t-a entries in diff

Entries added by "git add -N" are reminder for the user so that they
don't forget to add them before committing. These entries appear in
the index even though they are not real. Their presence in the index
leads to a confusing "git status" like this:

    On branch master
    Changes to be committed:
            new file:   foo

    Changes not staged for commit:
            modified:   foo

If you do a "git commit", "foo" will not be included even though
"status" reports it as "to be committed". This patch changes the
output to become

    On branch master
    Changes not staged for commit:
            new file:   foo

    no changes added to commit

The two hunks in diff-lib.c adjust "diff-index" and "diff-files" so
that i-t-a entries appear as new files in diff-files and nothing in
diff-index.

Due to this change, diff-files may start to report "new files" for the
first time. "add -u" needs to be told about this or it will die in
denial, screaming "new files can't exist! Reality is wrong." Luckily,
it's the only one among run_diff_files() callers that needs fixing.

Now in the new world order, a hierarchy in the index that contain
i-t-a paths is written out as a tree object as if these i-t-a
entries do not exist, and comparing the index with such a tree
object that would result from writing out the hierarchy will result
in no difference.  Update a test in t2203 that expected the i-t-a
entries to appear as "added to the index" in the comparison to
instead expect no output.

An earlier change eec3e7e4 (cache-tree: invalidate i-t-a paths after
generating trees, 2012-12-16) becomes an unnecessary pessimization
in the new world order---a cache-tree in the index that corresponds
to a hierarchy with i-t-a paths can now be marked as valid and
record the object name of the tree that results from writing a tree
object out of that hierarchy, as it will compare equal to that tree.

Reverting the commit is left for the future, though, as it is purely
a performance issue and no longer affects correctness.

Helped-by: Michael J Gruber <git@drmicha.warpmail.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>