fix(fs.lua): normalize slash truncation

Preserve last slash in windows' root drive directories

(cherry picked from commit 886996ff74)

runtime/lua/vim/fs.lua

@@ -344,7 +344,11 @@ function M.normalize(path, opts)
     path = path:gsub('%$([%w_]+)', vim.loop.os_getenv)
   end
 
-  return (path:gsub('\\', '/'):gsub('/+', '/'):gsub('(.)/$', '%1'))
+  path = path:gsub('\\', '/'):gsub('/+', '/')
+  if iswin and path:match('^%w:/$') then
+    return path
+  end
+  return (path:gsub('(.)/$', '%1'))
 end
 
 return M

test/functional/lua/fs_spec.lua

@@ -290,5 +290,10 @@ describe('vim.fs', function()
         return vim.fs.normalize('$XDG_CONFIG_HOME/nvim')
       ]], xdg_config_home))
     end)
+    if is_os('win') then
+      it('Last slash is not truncated from root drive', function()
+        eq('C:/', exec_lua [[ return vim.fs.normalize('C:/') ]])
+      end)
+    end
   end)
 end)