Last updated April 22, 2016.
With all the names, usernames, git usernames, user.names and e-mail around, let's take a look at what they mean and where they appear.
The username set at http://drupal.org/user which you use to log into Drupal.org is, as far as it relates to Git, displayed:
On the Commitlog
On the View All Committers page
On the Maintainer's block
Your Drupal.org password, set at http://drupal.org/user, is both your Drupal.org password AND your Git password.
The Git username, set permanently at http://git-dev.drupal.org/user/[uid]/edit/git, is used for Git authentication when communicating with the server. This is typically the same as your Drupal.org username, but can be different for older accounts due to historical reasons. You'll see it in:
On the Git access tab
Drupal.org e-mail address(es)
Any e-mail address associated with your Drupal.org account is also a valid choice for a Git e-mail address. Note that while an e-mail address is associated with your account it cannot be used by any other account. These e-mail addresses aren't visible anywhere on Drupal.org, except to you on the "E-mail addresses" tab.
(Your name, just like the "friendly name" displayed on an e-mail.)
This shows with user.email in the normal git commit log locally.
And it shows the repository viewer (without the e-mail).
To get credit for a commit, the e-mail address you use here must match one of the e-mail addresses you've confirmed. This e-mail address will be visible in the repository's git log, but is not exposed to the web (and spam bots) anywhere on Drupal.org. In addition, you can choose to use a fake address. The format is [git-username]@[your-uid].no-reply.drupal.org