Append -- to git checkout commands

Checking out a branch or tag may not succeed if there is a path entry
with the same name. This commit appends -- to git checkout commands to
clarify that we're not referring to a file or a directory.

Close #602

test/run

@@ -95,6 +95,8 @@ DOC
   echo 'let g:foo = 1' > plugin/foo.vim
   git add plugin/foo.vim
   git commit -m initial
+  git checkout -b plugin
+  git checkout master
 
   cd "$BASE"
 }