ReviewBoard with git. "There don't seem to be any diffs!" but there are

Question

I'm trying this codereview tool. I'm having problems with git. This is the output of git status:

# On branch master
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   path/to/something/modified.js
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   .gitignore
#   .reviewboardrc

If i try to run post-review --guess-summary --guess-description -p under this status I get this message:

There don't seem to be any diffs!

Now, if I run git add -u, my git status is:

# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   modified:   path/to/something/modified.js
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   .gitignore
#   .reviewboardrc

With this situation, if I run again post-review --guess-summary --guess-description -p I still get the same message:

There don't seem to be any diffs!

At this point if I commit it with the command git commit -m "my commit" I have the following message (obviously):

[master 027e044] my commit 1 file changed, 1 insertion(+)

and I run again the post-review command I have the output:

Review request #22 posted.

http://192.168.133.218/r/22/

This puzzles me because If you go to read the ReviewBoard FAQ here's the message:

A basic pre-commit workflow with git would look like this:

Clone the central repository.
Make a change you want reviewed, but do not commit it yet.
Run post-review (or otherwise submit a diff).
Get reviews, update your change as needed.
When the change is marked to ship, commit it to master and push it to the origin.

Since my intention was doing a pre-commit, can you explain to me why there are no diff after I stage changes for commit?