Fix regression in chdir when switching drives.

Fix WindowsChdirTestCase.test_windows_getcwd_ensures_drive_letter in
test.test_chdir, completing #2307. (Previously-failing tests pass on
both Windows and Linux.)

Author: jeff5

src/org/python/modules/posix/PosixModule.java

@@ -1144,12 +1144,18 @@ private static Path absolutePath(PyObject pathObj) {
         }
         try {
             Path path = Paths.get(pathStr);
-            if (!path.isAbsolute()) {
-                // Relative path: augment from current working directory.
-                path = Paths.get(Py.getSystemState().getCurrentWorkingDir()).resolve(path);
-            }
+            // Relative path: augment from current working directory.
+            path = Paths.get(Py.getSystemState().getCurrentWorkingDir()).resolve(path);
+            // In case of a root different from cwd, resolve does not guarantee absolute.
+            path = path.toAbsolutePath();
             // Strip redundant navigation a/b/../c -> a/c
-            return path.normalize();
+            path = path.normalize();
+            // Prevent trailing slash (possibly Java bug), except when '/' or C:\
+            pathStr = path.toString();
+            if (pathStr.endsWith(path.getFileSystem().getSeparator()) && path.getNameCount()>0) {
+                path = Paths.get(pathStr.substring(0, pathStr.length()-1));
+            }
+            return path;
         } catch (java.nio.file.InvalidPathException ex) {
             /*
              * Thrown on Windows for paths like foo/bar/<test>, where <test> is the literal text,