How do I undo 'git add' before commit?

I mistakenly added files to Git using the command:

git add myfile.txt

I have not yet run git commit. How do I undo this so that these changes will not be included in the commit?

To unstage a specific file

git reset <file>

That will remove the file from the current index (the "about to be committed" list) without changing anything else.

To unstage all files from the current change set:

git reset

In old versions of Git, the above commands are equivalent to git reset HEAD <file> and git reset HEAD respectively, and will fail if HEAD is undefined (because you haven't yet made any commits in your repository) or ambiguous (because you created a branch called HEAD, which is a stupid thing that you shouldn't do). This was changed in Git 1.8.2, though, so in modern versions of Git you can use the commands above even prior to making your first commit:

"git reset" (without options or parameters) used to error out when you do not have any commits in your history, but it now gives you an empty index (to match non-existent commit you are not even on).

Documentation: git reset