unstage

git diff of files that have been staged ie 'git add'ed
git diff --cached

http://www.commandlinefu.com/commands/view/1242/git-diff-of-files-that-have-been-staged-ie-git-added

$ git commit ...
$ git reset --soft HEAD^ (1)
$ edit (2)

And just commit again.

git reset filename.txt

Will remove a file named filename.txt from the current index, the "about to be committed" area, without changing anything else.

To undo git add . use git reset (no dot).