FsDiscoverer: Fix invalid folder comparison

In case the devices containing the folder have multiple mountpoints,
using the MRL might lead to false negative.
Should fix #125
5 jobs for master in 32 minutes and 30 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #304789
amd64 debian
asan-ubsan

00:14:10

passed #304788
amd64 debian
coverage

00:18:18

80.0%
passed #304785
amd64 debian
linux

00:18:22

passed #304786
win32
win32

00:22:33

passed #304787
win64
win64

00:21:45