Comment by diggan

Different gitconfigs per path on disk, git automatically uses the right identity depending on where you are on disk. Allows you to separate all projects into different users and every repository within those directories will use the specified user.

Main/default config (~/.gitconfig):

  [user]
    email = [email protected]
    name = Victor Bjelkholm

  [includeIf "gitdir:/home/user/projects/user-a/"]
    path = /home/user/.gitconfig-user-a

  [includeIf "gitdir:/home/user/projects/user-b/"]
    path = /home/user/.gitconfig-user-b

Then in .gitconfig-user-{a,b}:

  [user]
    name = UserA
    email = [email protected]

  [core]
    sshCommand = ssh -i /home/user/.ssh/user_a_id_ed25519

Or, even more separation, create separate users/accounts on the computers you use.