[functoid]

git config include.path "~/.config/git/id_example"

  ; .config/git/id_example
  [user]
    name = "git"
    email = "git@localhost"
  [core]
    sshCommand = "ssh -i ~/.ssh/id_example"

Configurable per repo with many identities. Add a global alias to engage any identity at any time.

git config --global alias.as-example '!git -c include.path=~/.config/git/id_example'

  git as-example commit -m "my commit message as example"