QFileDialog wrongly strips filename prefix when accessing parent dir

Original report: https://github.com/qutebrowser/qutebrowser/issues/7150

When the current folder is, say xyz, and the string xyzc (with c being any character, apparently) matches the beginning of sibling files/directories' names, those files/dirs are not selectable by writing ../xyz and hitting DownArrow/UpArrow, because when one is selected, the ../xyzc prefix is stripped off.

See the GIF:

This is reproducible by e.g. doing touch memgraph.mp3 memgraph.git memgraphmp3; mkdir memgraph in an empty directory, calling QFileDialog::getOpenFileName(), navigating to the memgraph subfolder, and then entering ../memgraph and pressing down arrow (like in the gif).