fix(termopen): avoid ambiguity in URI when CWD is root dir (#16988)

test/functional/ex_cmds/mksession_spec.lua

@@ -1,5 +1,6 @@
 local lfs = require('lfs')
 local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
 
 local clear = helpers.clear
 local command = helpers.command
@@ -119,4 +120,45 @@ describe(':mksession', function()
     command('bd!')
     sleep(100)  -- Make sure the process exits.
   end)
+
+  it('restores CWD for :terminal buffer at root directory #16988', function()
+    if helpers.iswin() then
+      pending('N/A for Windows')
+      return
+    end
+
+    local screen
+    local cwd_dir = funcs.fnamemodify('.', ':p:~'):gsub([[[\/]*$]], '')
+    local session_path = cwd_dir..'/'..session_file
+
+    screen = Screen.new(50, 6)
+    screen:attach({rgb=false})
+    local expected_screen = [[
+      ^/                                                 |
+                                                        |
+      [Process exited 0]                                |
+                                                        |
+                                                        |
+                                                        |
+    ]]
+
+    command('cd /')
+    command('terminal echo $PWD')
+
+    -- Verify that the terminal's working directory is "/".
+    screen:expect(expected_screen)
+
+    command('cd '..cwd_dir)
+    command('mksession '..session_path)
+    command('qall!')
+
+    -- Create a new test instance of Nvim.
+    clear()
+    screen = Screen.new(50, 6)
+    screen:attach({rgb=false})
+    command('silent source '..session_path)
+
+    -- Verify that the terminal's working directory is "/".
+    screen:expect(expected_screen)
+  end)
 end)