cli/tests: Prevent git.subprocess from reading outside git config

A subset of cli tests could fail if the system /etc/gitconfig had
configuration interfering with the tests. The cause seems to be running
of `jj` commands that would in turn use a `git` subprocess.

Fix this by setting `GIT_CONFIG_SYSTEM` and `GIT_CONFIG_GLOBAL`, like
in `hermetic_git`.

Fixes #6159

cli/tests/common/test_environment.rs

···
         // executables like `git` from the PATH.
         cmd.env("PATH", std::env::var_os("PATH").unwrap_or_default());
         cmd.env("HOME", self.home_dir.to_str().unwrap());
+        // Prevent git.subprocess from reading outside git config
+        cmd.env("GIT_CONFIG_SYSTEM", "/dev/null");
+        cmd.env("GIT_CONFIG_GLOBAL", "/dev/null");
         cmd.env("JJ_CONFIG", self.config_path.to_str().unwrap());
         cmd.env("JJ_USER", "Test User");
         cmd.env("JJ_EMAIL", "[e-mail]");

cli/tests/test_git_remotes.rs

···
         .open(work_dir.root().join(".jj/repo/store/git/config"))
         .unwrap();
     // `git clone` adds branch configuration like this.
-    writeln!(config_file, "[branch \"test\"]").unwrap();
-    writeln!(config_file, "\tremote = foo").unwrap();
-    writeln!(config_file, "\tmerge = refs/heads/test").unwrap();
+    let eol = if cfg!(windows) { "\r\n" } else { "\n" };
+    write!(config_file, "[branch \"test\"]{eol}").unwrap();
+    write!(config_file, "\tremote = foo{eol}").unwrap();
+    write!(config_file, "\tmerge = refs/heads/test{eol}").unwrap();
     drop(config_file);
 
     let output = work_dir.run_jj(["git", "remote", "rename", "foo", "bar"]);