0> git init
Initialized empty Git repository in /Users/chrisb/git_test/.git/
0> echo "some\ncontent" >> foo.txt
0> git add .
0> git commit -m 'first'
[master (root-commit) 3e1a325] first
 1 file changed, 2 insertions(+)
 create mode 100644 foo.txt
0> git diff-index HEAD
0> touch foo.txt
0> git diff-index HEAD
:100644 100644 6cfc2af7f5fbb8cd7cb226e7a70d1829a0678767 0000000000000000000000000000000000000000 M	foo.txt
0> git status
On branch master
nothing to commit, working tree clean
0> git diff-index HEAD
0>