Log Message:
Add git-credential-osxkeychain: Git macOS Keychain credential manager

git-credential-osxkeychain caches credentials in the secure keychain
that's attached to your macOS system account. This method stores the
credentials on disk, and they never expire, but they're encrypted with
the same system that stores HTTPS certificates and Safari auto-fills.

To use with all your repos:

$ git config --global credential.helper osxkeychain