mksession: restore same :term buf in split windows

Problem: When session-restore creates a terminal buffer with command
         like `:edit term://.//16450:/bin/bash`, the buffer gets
         a different name (depends on PID). Thus the later call to
         `bufexists('term://.//16450:/bin/bash)` will return false.
Solution: Force the buffer name with :file. This as least ensures
         the same buffer will show in multiple windows correctly, as
         expected when saving the session.  But it still has problems:
           1. the PID in the buffer name is bogus
           2. redundant :terminal buffers still hang around

fix #5250

test/functional/ex_cmds/mksession_spec.lua

@@ -91,4 +91,23 @@ describe(':mksession', function()
     matches('^term://'..pesc(expected_cwd)..'//%d+:', funcs.expand('%'))
     command('qall!')
   end)
+
+	it('restores multiple windows with same terminal instances', function()
+		-- Create a view with two buffers referencing the same terminal instance
+		command('terminal')
+		command('split')
+		command('mksession ' .. session_file)
+
+		clear()
+
+		command('source ' .. session_file)
+		-- Getting the name of the buffer shown to compare with the other window
+		local eval = helpers.eval
+
+		command('exe 1 . "wincmd w"')
+		local expected_pid = eval('b:terminal_job_pid')
+
+		command('exe 2 . "wincmd w"')
+		eq(expected_pid, eval('b:terminal_job_pid'))
+	end)
 end)