Fix T68000: load previous settings error if config folder already exists

This happened when opening Blender and not clicking Load Previous Settings
or Next, and then reopening Blender again and using Load Previous Settings.

release/scripts/startup/bl_operators/userpref.py

@@ -49,6 +49,44 @@ def module_filesystem_remove(path_base, module_name):
             else:
                 os.remove(f_full)
 
+# This duplicates shutil.copytree from Python 3.8, with the new dirs_exist_ok
+# argument that we need. Once we upgrade to 3.8 we can remove this.
+def _preferences_copytree(entries, src, dst):
+    import shutil
+    import os
+    os.makedirs(dst, exist_ok=True)
+    errors = []
+
+    for srcentry in entries:
+        srcname = os.path.join(src, srcentry.name)
+        dstname = os.path.join(dst, srcentry.name)
+        srcobj = srcentry
+        try:
+            if srcentry.is_symlink():
+                linkto = os.readlink(srcname)
+                os.symlink(linkto, dstname)
+                shutil.copystat(srcobj, dstname, follow_symlinks=False)
+            elif srcentry.is_dir():
+                preferences_copytree(srcobj, dstname)
+            else:
+                shutil.copy2(srcentry, dstname)
+        except Error as err:
+            errors.extend(err.args[0])
+        except OSError as why:
+            errors.append((srcname, dstname, str(why)))
+    try:
+        shutil.copystat(src, dst)
+    except OSError as why:
+        if getattr(why, 'winerror', None) is None:
+            errors.append((src, dst, str(why)))
+    if errors:
+        raise Error(errors)
+    return dst
+
+def preferences_copytree(src, dst):
+    import os
+    with os.scandir(src) as entries:
+        return _preferences_copytree(entries=entries, src=src, dst=dst)
 
 class PREFERENCES_OT_keyconfig_activate(Operator):
     bl_idname = "preferences.keyconfig_activate"
@@ -110,9 +148,10 @@ class PREFERENCES_OT_copy_prev(Operator):
         return os.path.isfile(old_userpref) and not os.path.isfile(new_userpref)
 
     def execute(self, _context):
-        import shutil
-
-        shutil.copytree(self._old_path(), self._new_path(), symlinks=True)
+        # Use this instead once we upgrade to Python 3.8 with dirs_exist_ok.
+        # import shutil
+        # shutil.copytree(self._old_path(), self._new_path(), dirs_exist_ok=True)
+        preferences_copytree(self._old_path(), self._new_path())
 
         # reload preferences and recent-files.txt
         bpy.ops.wm.read_userpref()