Why the `git commit -am` command is working with untracked file (in the documentation's samples)?

Question

Often in the Git documentation I see something like this:

$ git clone john@githost:simplegit.git
Initialized empty Git repository in /home/john/simplegit/.git/
...
$ cd simplegit/
$ vim lib/simplegit.rb
$ git commit -am 'removed invalid default value'
[master 738ee87] removed invalid default value
 1 files changed, 1 insertions(+), 1 deletions(-)

From the documentation:

-a
--all

Tell the command to automatically stage files that have been modified and deleted, but new files you have not told Git about are not affected.

Pay attention, the lib/simplegit.rb file wasn't tracked when it was created at once. Whay the -a option does it tracked? I use Git for Windows and this sample doesn't work when I try to do the similar:

bushm@DESKTOP-ISD2NUH MINGW64 /d/src/sample
$ git init
Initialized empty Git repository in D:/src/sample/.git/

bushm@DESKTOP-ISD2NUH MINGW64 /d/src/sample (master)
$ vim ./text.txt

bushm@DESKTOP-ISD2NUH MINGW64 /d/src/sample (master)
$ git commit -am "First commit"
On branch master

Initial commit

Untracked files:
        text.txt

nothing added to commit but untracked files present

bushm@DESKTOP-ISD2NUH MINGW64 /d/src/sample (master)

Answer 1

Update: the part of the documentation that made this confusing was fixed in PR #414 by Matthieu Moy

---

This is probably just a mistake in the example.

Firstly, you are right. -a only adds modified and deleted files.

Secondly, when you clone a repo, git doesn't say

Initialized empty Git repository in /home/john/simplegit/.git/

instead it says

Cloning into 'simplegit'...
remote: Counting objects

and some other lines.

Thirdly, in the example

# John's Machine
$ git clone john@githost:simplegit.git
Initialized empty Git repository in /home/john/simplegit/.git/
...
$ cd simplegit/
$ vim lib/simplegit.rb
$ git commit -am 'removed invalid default value'
[master 738ee87] removed invalid default value
 1 files changed, 1 insertions(+), 1 deletions(-)

you can see the commit message is 'removed invalid default value', which implies that it is modifying a tracked file.

I think the author is just combining some things so he can show a clear example.

It would have been better if he had left out the line Initialized empty Git repository in /home/john/simplegit/.git/