Bugfix: Save files with invalid timestamps

When restic reads invalid timestamps (year is before 0000 or after 9999) it
refused to read and archive the file. We've changed the behavior and will now
save modified timestamps with the year set to either 0000 or 9999, the rest of
the timestamp stays the same, so the file will be saved (albeit with a bogus
timestamp).

https://github.com/restic/restic/issues/2174
https://github.com/restic/restic/issues/1173
