Always pass in the working directory in path_get_cdpath

If the user is in a directory which has been unlinked, it is possible
for the path .. to not exist, relative to the working directory.
Always pass in the working directory (potentially virtual) to
path_get_cdpath; this ensures we check absolute paths and are immune
from issues if the working directory has been unlinked.

Also introduce a new function path_normalize_for_cd which normalizes the
"join point" of a path and a working directory. This allows us to 'cd' out of
a non-existent directory, but not cd into such a directory.

Fixes #5341

src/parse_execution.cpp

@@ -809,7 +809,8 @@ parse_execution_result_t parse_execution_context_t::populate_plain_process(
                 !args.try_get_child<g::redirection, 0>()) {
                 // Ok, no arguments or redirections; check to see if the command is a directory.
                 wcstring implicit_cd_path;
-                use_implicit_cd = path_can_be_implicit_cd(cmd, &implicit_cd_path);
+                use_implicit_cd =
+                    path_can_be_implicit_cd(cmd, env_get_pwd_slash(), &implicit_cd_path);
             }
         }