git-extras: rewrite test

It feels a bit weird with our big push towards sandboxing tests to let things
deliberately cd outside of the testpath to make the test function.

Since that isn't necessary here, a simple rewrite achieves the same thing.

Formula/git-extras.rb

@@ -3,7 +3,6 @@ class GitExtras < Formula
   homepage "https://github.com/tj/git-extras"
   url "https://github.com/tj/git-extras/archive/3.0.0.tar.gz"
   sha256 "490742428824d6e807e894c3b6612be37a9a9a4e8fbea747d1813e5d62b2a807"
-
   head "https://github.com/tj/git-extras.git"
 
   bottle do
@@ -19,8 +18,7 @@ class GitExtras < Formula
   end
 
   test do
-    cd HOMEBREW_PREFIX do
-      system "#{bin}/git-root"
-    end
+    system "git", "init"
+    assert_match /#{testpath}/, shell_output("#{bin}/git-root")
   end
 end