duplicity: don't shell out to move files

And don't invoke system in such a way that escapes the glob.

Fixes Homebrew/homebrew#10196.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/duplicity.rb

@@ -18,8 +18,8 @@ class Duplicity < Formula
                      "--install-scripts=#{bin}"
 
     # Shift files around to avoid needing a PYTHONPATH
-    system "mv", "#{bin}/duplicity", "#{bin}/duplicity.py"
-    system "mv", "#{bin}/*", libexec
+    mv bin+'duplicity', bin+'duplicity.py'
+    mv Dir[bin+'*'], libexec
 
     bin.install_symlink "#{libexec}/duplicity.py" => "duplicity"
     bin.install_symlink "#{libexec}/rdiffdir"