brew-unpack: don't attempt to mv the build directory

When the current working directory is the root of the build tree, copy
it instead of moving it, otherwise an exception will be raised when we
try to clean up the build tree later.

Noticed when unpacking the vim source tree from a mercurial checkout.

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

Library/Contributions/cmds/brew-unpack.rb

@@ -90,7 +90,7 @@ changes.
       ENV['VERBOSE'] = '1' # show messages about tar
       f.brew do
         cd Dir['*'][0] if Dir['*'].one?
-        mv getwd, stage_dir
+        cp_r getwd, stage_dir
       end
       ENV['VERBOSE'] = nil