go up from a path that ends in a separator

[SashaMIT]

Summary

RemotePath::parent of /home/user/ returned /home/user. The trailing separator was counted as a directory level, so one step up stayed in the same directory. /home/ returned /home. C:\Users\ returned C:\Users. C:\Users\npace\ returned C:\Users\npace.

The directory picker uses this for the up action. A trailing separator is not a level. /home/user/ is now /home. /home/ is now /. C:\Users\ is now C:\. C:\Users\npace\ is now C:\Users. /home/user is still /home. C:\Users\npace is still C:\Users. C:\ is still C:\.

Test plan

• parent_trailing_separator_is_not_a_level fails on tip (/home/user/ stays /home/user) and passes after the fix
• existing parent_posix, parent_windows, and parent_windows_forward_slashes still pass, along with the rest of the remote_path tests