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/regressions.vader

@@ -341,3 +341,16 @@ Execute (#593 Add plugin to &rtp before running post-update hook with : prefix):
   call plug#end()
   PlugInstall!
   AssertEqual ['a', 'b'], g:bar
+
+**********************************************************************
+Execute (#602 Confusion with branch name and path name):
+  call plug#begin()
+  Plug expand('file:////tmp/vim-plug-test/new-branch'), { 'branch': 'plugin' }
+  call plug#end()
+  PlugUpdate
+  call PlugStatusSorted()
+
+Expect:
+  - new-branch: OK
+  Finished. 0 error(s).
+  [=]