"Could not open foo.squashfs: No such file or directory" when comparing filesystems

This bug was originally reported by Paul Wise (pabs@debian.org) in Debian bug #964365:

Package: diffoscope
Version: 150
Severity: normal

When comparing squashfs filesystems I get an error saying the first
filesystem could not be found, even though I created it just before.=20

$ mkdir foo bar
$ touch foo/foo bar/foo
$ chronic mksquashfs foo foo.squashfs
$ chronic mksquashfs bar bar.squashfs
$ diffoscope foo.squashfs bar.squashfs=20
--- foo.squashfs
+++ bar.squashfs
=E2=94=82=E2=94=84 Command `unsquashfs -n -f -no -li -d . foo.squashfs` fai=
led with exit code 1. Standard error:
=E2=94=82=E2=94=84     Could not open foo.squashfs, because No such file or=
 directory
@@ -1,8 +1,8 @@
-00000000: 6873 7173 0200 0000 eeac 025f 0000 0200  hsqs......._....
+00000000: 6873 7173 0200 0000 f1ac 025f 0000 0200  hsqs......._....
 00000010: 0000 0000 0100 1100 c000 0100 0400 0000  ................
 00000020: 2000 0000 0000 0000 c700 0000 0000 0000   ...............
 00000030: bf00 0000 0000 0000 ffff ffff ffff ffff  ................
 00000040: 6000 0000 0000 0000 8c00 0000 0000 0000  `...............
 00000050: a100 0000 0000 0000 b100 0000 0000 0000  ................
 00000060: 2a00 78da 6362 58c0 c800 044f d730 c583  *.x.cbX....O.0..
 00000070: 1940 f01f 08a0 4c06 4686 1770 7926 a818  .@....L.F..py&..