Fix upper/lower case rename on case insensitive fs

Summary:
Fixes wrong temp file creation (missing '/' at
relative root of mounted fs).

Test Plan:
Mount a CIFS share e.g. at /mnt/share with option
'nocase'. In dolphin, go there and create a file
'foo' (i.e. /mnt/share/foo).
Try to rename it to 'Foo': Without this patch a
dialog pops up telling you about overwriting the file.
With this patch applied it will correctly be renamed.

Create a directory 'subdir' below /mnt/share.
In dolphin, change there and create a file 'foo'
(i.e. /mnt/share/subdir/foo).
Try to rename it to 'Foo': no problem with and without
this patch.

Reviewers: Frameworks, Dolphin, dfaure, chinmoyr

Reviewed By: dfaure, chinmoyr

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19422