[git] Git keeps asking me for my ssh key passphrase

This has been happening to me after restarts since upgrading from OS X El Capitan (10.11) to macOS Sierra (10.12). The ssh-add solution worked temporarily but would not persist across another restart.

The permanent solution was to edit (or create) ~/.ssh/config and enable the UseKeychain option.

Host *
    UseKeychain yes

Related: macOS keeps asking my ssh passphrase since I updated to Sierra