I found this user management strategy somewhere, and it's been working great for me:

  git config --global --unset user.name
  git config --global --unset user.email
  git config --global --unset user.signingkey

  git config --global user.useConfigOnly true

  git config --global user.<id>.name "<name>"
  git config --global user.<id>.email "<email>"

  git config --global alias.identity '! git config user.name "$(git config user.$1.name)"; git config user.email "$(git config user.$1.email)"; :'

So given that I have created two users, e.g. personal and work I run:

  git identity work

in repos that need the work name/e-mail, and

  git identity personal

in the ones that are private.