git add -N new_file
git add -i

-N, --intent-to-add
    Record only the fact that the path will be added later.  An entry
    for the path is placed in the index with no content.  This is useful
    for, among other things, showing the unstaged content of such files
    with git diff and committing them with git commit -a.

git update-index --add --cacheinfo 100644 $(git hash-object -w /dev/null) newfile
git add --interactive newfile

mkdir /tmp/demo
cd /tmp/demo
git init .

echo hello > newfile
git update-index --add --cacheinfo 100644 $(git hash-object -w /dev/null) newfile

$ find .git/objects/ -type f
.git/objects/e6/9de29bb2d1d6434b8b29ae775ad8c2e48c5391

$ git status
# On branch master
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#
#   new file:   newfile
#
# Changed but not updated:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   newfile
#

$ git diff
diff --git a/newfile b/newfile
index e69de29..ce01362 100644
--- a/newfile
+++ b/newfile
@@ -0,0 +1 @@
+hello

git update-index --add --cacheinfo 100644 $(git hash-object -w /dev/null) newfile

git add -p # (or --patch)