Run File::Find::find with no_chdir=>1

Without no_chdir=>1 the unshared child process in unshare mode needs
read permissions for the directory from which mmdebstrap is executed.
With this change, the current working directory does not need to be
world-readable anymore.

Closes: #1005857
Reported-by: Trent W. Buck <trentbuck@gmail.com>

README.md

@@ -154,3 +154,4 @@ Contributors
  - Konstantin Demin
  - Trent W. Buck
  - Vagrant Cascadian
+ - Gioele Barabucci

mmdebstrap

@@ -4094,7 +4094,10 @@ sub approx_disk_usage {
             $installed_size += 1;
         }
     };
-    find($scan_installed_size, $directory);
+    # We use no_chdir because otherwise the unshared user has to have read
+    # permissions for the current working directory when producing an ext2
+    # image. See https://bugs.debian.org/1005857
+    find({ wanted => $scan_installed_size, no_chdir => 1 }, $directory);
 
     # because the above is only a heuristic we add 10% extra for good measure
     return int($installed_size * 1.1);
@@ -6817,12 +6820,14 @@ Create a bootable USB Stick that boots into a full Debian desktop:
     END
     # You can use $(sudo blockdev --getsize64 /dev/sdXXX) to get the right
     # image size for the target medium in bytes
-    $ guestfish -N debian-unstable.img=disk:8G -- part-disk /dev/sda mbr : \
-        part-set-bootable /dev/sda 1 true : mkfs ext2 /dev/sda1 : \
-        set-label /dev/sda1 rootfs : mount /dev/sda1 / : \
+    $ guestfish -N debian-unstable.img=disk:8G -- \
+        part-disk /dev/sda mbr : \
+        part-set-bootable /dev/sda 1 true : \
+        set-label /dev/sda1 rootfs : \
+        mkfs ext4 /dev/sda1 : mount /dev/sda1 / : \
         tar-in debian-unstable.tar / xattrs:true : \
-        upload /usr/lib/SYSLINUX/mbr.bin /mbr.bin : \
-        copy-file-to-device /mbr.bin /dev/sda size:440 : rm /mbr.bin : \
+        upload /usr/lib/EXTLINUX/mbr.bin /boot/mbr.bin : \
+        copy-file-to-device /boot/mbr.bin /dev/sda size:440 : \
         extlinux / : copy-in extlinux.conf / : sync : umount / : shutdown
     $ qemu-system-x86_64 -m 1G -enable-kvm debian-unstable.img
     $ sudo dd if=debian-unstable.img of=/dev/sdXXX status=progress
@@ -6857,10 +6862,11 @@ Use as replacement for autopkgtest-build-qemu and vmdb2:
     $ guestfish -N debian-unstable.img=disk:8G -- \
         part-disk /dev/sda mbr : \
         part-set-bootable /dev/sda 1 true : \
-        mkfs ext2 /dev/sda1 : mount /dev/sda1 / : \
+        mkfs ext4 /dev/sda1 : mount /dev/sda1 / : \
         tar-in debian-unstable.tar / xattrs:true : \
-        extlinux / : copy-in extlinux.conf / : \
-        sync : umount / : shutdown
+        upload /usr/lib/EXTLINUX/mbr.bin /boot/mbr.bin : \
+        copy-file-to-device /boot/mbr.bin /dev/sda size:440 : \
+        extlinux / : copy-in extlinux.conf / : sync : umount / : shutdown
     $ qemu-img convert -O qcow2 debian-unstable.img debian-unstable.qcow2
 
 As a debootstrap wrapper to run it without superuser privileges but using Linux