cd: fix path when trying to cd out of the root directory

Part of #12704

src/path.rs

@@ -358,7 +358,14 @@ pub fn path_apply_cdpath(dir: &wstr, wd: &wstr, env_vars: &dyn Environment) -> V
             // We want to return an absolute path (see issue 6220)
             if ![Some('/'), Some('~')].contains(&path.chars().next()) {
                 abspath = wd.to_owned();
-                abspath.push('/');
+
+                // Do not add a second slash if `wd` already ends with one
+                // (typically, when it's the root directory).
+                // This could result in unwanted paths (e.g. `//<path>`, which
+                // on Windows, is a remote directory).
+                if abspath.chars().next_back() != Some('/') {
+                    abspath.push('/');
+                }
             }
             abspath.push_utfstr(&path);
 

tests/checks/cd.fish

@@ -407,3 +407,10 @@ cd (string repeat 4096 a)
 # CHECKERR: ^
 # CHECKERR: in function 'cd' with arguments '{{.*}}'
 # CHECKERR: called on line {{\d+}} of file {{.*}}/cd.fish
+
+# Ensures `cd` doesn't create `<pwd>+/+<dir>` internally, when pwd is `/`, i.e.
+# results in `//<dir>`. This test will (hopefully) fail on platforms where such
+# a path has a special meaning (e.g. Windows would fail trying to access a server
+# named "bin")
+cd /
+cd bin