How can you undo the last git add?

You can use git reset. This will 'unstage' all the files you've added after your last commit.

If you want to unstage only some files, use git reset -- <file 1> <file 2> <file n>.

Also it's possible to unstage some of the changes in files by using git reset -p.

So the real answer to

Can this programmer now unstage his last changes with some magical git command?

is actually: No, you cannot unstage just the last git add.

That is if we interpret the question as in the following situation:

Initial file:

void foo() {

}

main() {
    foo();
}

First change followed by git add:

void foo(int bar) {
    print("$bar");
}

main() {
    foo(1337);
}

Second change followed by git add:

void foo(int bar, String baz) {
    print("$bar $baz");
}

main() {
    foo(1337, "h4x0r");
}

In this case, git reset -p will not help, since its smallest granularity is lines. git doesn't know that about the intermediate state of:

void foo(int bar) {
    print("$bar");
}

main() {
    foo(1337);
}

any more.

You cannot undo the latest git add, but you can undo all adds since the last commit. git reset without a commit argument resets the index (unstages staged changes):

git reset