pre-commit: improve test (#21443)

Now covers:
- `PYTHONPATH` based setup
- relative virtualenv symlinks

Formula/pre-commit.rb

@@ -47,7 +47,16 @@ class PreCommit < Formula
             -   id: trailing-whitespace
       EOS
       system bin/"pre-commit", "install"
-      system bin/"pre-commit", "run", "--all-files"
+      (testpath/"f").write "hi\n"
+      system "git", "add", "f"
+
+      ENV["GIT_AUTHOR_NAME"] = "test user"
+      ENV["GIT_AUTHOR_EMAIL"] = "test@example.com"
+      ENV["GIT_COMMITTER_NAME"] = "test user"
+      ENV["GIT_COMMITTER_EMAIL"] = "test@example.com"
+      git_exe = which("git")
+      ENV["PATH"] = "/usr/bin:/bin"
+      system git_exe, "commit", "-m", "test"
     end
   end
 end