I just Git init'ed a repos with a wrong user, and want to undo it. Is there any command for this? Do I actually have to go in and edit the .git directory?
If you just inited it, you can just delete .git.
Typically:
rm -rf .git
Then, recreate as the right user.
EDIT: NOTE: This deletes the entire git repository (including all commit history, etc.). On a newly-created repository, there is no history yet.