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).
+  [=]

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"
 }

test/workflow.vader

@@ -1473,7 +1473,7 @@ Execute (Commit hash support):
   PlugUpdate
   Log getline(1, '$')
   AssertEqual 'x goyo.vim:', getline(5)
-  AssertEqual '    error: pathspec ''ffffffff'' did not match any file(s) known to git.', getline(6)
+  AssertEqual '    fatal: invalid reference: ffffffff', getline(6)
   AssertEqual 0, stridx(getline(7), '- vim-emoji: HEAD is now at 9db7fcf...')
 
   let hash = system(printf('cd %s && git rev-parse HEAD', g:plugs['vim-emoji'].dir))[:-2]