Bugfix: Use ctime when checking for file changes

Previously, restic only checked a file's mtime (along with other non-timestamp
data) to decide if a file has changed. This could cause it to not notice changes
if something edits a file and then resets the timestamp. Restic now also checks
the ctime, so any modification to a file should be noticed.

https://github.com/restic/restic/issues/2179