wrealpath: Fail for file/something

This incorrectly allowed "file/something" if file existed (as a file), because it
checked "afile".

Fixes #5352.

src/wutil.cpp

@@ -421,7 +421,9 @@ maybe_t<wcstring> wrealpath(const wcstring &pathname) {
             } else {
                 errno = 0;
                 // Only call realpath() on the portion up to the last component.
-                narrow_res = realpath(narrow_path.substr(0, pathsep_idx).c_str(), tmpbuff);
+                // Be sure to include the last "/", so that the penultimate component is considered as a directory.
+                // Otherwise "file/something" succeeds.
+                narrow_res = realpath(narrow_path.substr(0, pathsep_idx + 1).c_str(), tmpbuff);
 
                 if (!narrow_res) return none();
 

tests/realpath.err

@@ -1,2 +1,3 @@
 builtin realpath: /this/better/be/an/invalid/path: No such file or directory
 builtin realpath: .: No such file or directory
+builtin realpath: realpath.out/..: Not a directory

tests/realpath.in

@@ -80,4 +80,7 @@ else
     echo "unexpected pwd-resolved-to-itself failure: $real_path != $PWD" >&2
 end
 
+# Confirm that the penultimate component still needs to be a directory:
+builtin realpath realpath.out/..
+
 exit 0