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
cf67f19ac2