Merge fails when git is not configured (i.e. run as root)

[Atemu]

I just tried running nixpkgs-review as root because I wanted to use a nix store on a different drive and ran into this:

$ git merge --no-commit --no-ff 927fc72f6a5e35823543b194b6aa14a57fcfc8b2
Committer identity unknown

*** Please tell me who you are.

Run

  git config --global user.email "[email protected]"
  git config --global user.name "Your Name"

to set your account's default identity.
Omit --global to set the identity only in this repository.

fatal: unable to auto-detect email address (got 'root@SOTERIA.(none)')
https://github.com/NixOS/nixpkgs/pull/331966 failed to build: Failed to merge 927fc72f6a5e35823543b194b6aa14a57fcfc8b2 into /root/.cache/nixpkgs-review/pr-331966/nixpkgs. git merge failed with exit code 128

nixpkgs-review should not depend on any git configuration and should just provide git with placeholders here as we don't actually care about the merge commit metadata here.