Update pathname during squashfs pre_scan

[darktemplarbasealt]

This change may be useful during debugging

[nigels-com]

I can can test these two changes since I happen to be actively working with sqashfs at the moment...
I'll follow up in a day or two.

[darktemplarbasealt]

This change is not noticeable to user of this module since parent_name is only used for recursively constructing pathname which is used in next level of call to pre_scan. But it may be useful if you are debugging or if correct parent_name or pathname would be used for something else.

I've tested this change only with python 3.7.

[matteomattei]

@nigels-com did you test the changes?
If they work feel free to squash and merge.