[includeIf "hasconfig:remote.*.url:gh-work:**/**"]
        path = ~/.gitconfig.d/gh-work.inc

Essentially, the name `gh-work` becomes the distinguishing element in both my ssh identity and my git identity, and I find this easier to think about.

Just keep a .gitconfig in your HOME with aliases for your identities. Then just after initializing/cloning the repo do git config-company or git config-personal

    er453r@r7:~$ cat ~/.gitconfig 
    [user]
        useConfigOnly = true
    [alias]
        config-personal = !echo CONFIG-PERSONAL && \
            git config --local user.email 'personal@email.com' && \
            git config --local user.name 'personal' && \
            git config --local core.sshCommand 'ssh -i ~/.ssh/id_rsa_personal'
        config-company = !echo OLD CONFIG-COMPANY && \
            git config --local user.email 'official@comapny.io' && \
            git config --local user.name 'Name Surname' && \
            git config --local core.sshCommand 'ssh -i ~/.ssh/id_rsa_company'

It was super unhelpful when trying to do version control forensics. But if I'm being generous, I think maybe he was trying to remind everyone that anyone can put anything in their identity config, and we shouldn't trust whatever is in there for all that much.

Just put this in your ~/.gitconfig (or ~/.config/git/personal as in the article)

  [core]
      sshCommand = /usr/bin/ssh -o IdentitiesOnly=yes -i ~/.ssh/IdentityFile2 -a

    Host customer-github
     Hostname github.com
     IdentityFile ~/.ssh/customer_rsa
     User git

Aside: I use NixOS with home-manager (on linux and mac), which makes this trivial [1]. Added the following lines to my home-manager config:

  programs.git = {
    enable = true;
    ...
    includes = [
      {
        condition = "hasconfig:remote.*.url:git@github.com:<work>/**";
        contents = {
          user.email = "<work email>";
        };
      }
    ];
  }