Enhancement: Add --ignore-inode option to backup cmd

This option handles backup of virtual filesystems that do not keep fixed
inodes for files, like Fuse-based, pCloud, etc. Ignoring inode changes allows
to consider the file as unchanged if last modification date and size
are unchanged.

https://github.com/restic/restic/pull/2205
https://github.com/restic/restic/pull/2047
https://github.com/restic/restic/issues/1631