Enhancement: Use ctime when checking for file changes

Previously, restic only checked a file's mtime (along with other non-timestamp
metadata) to decide if a file has changed. This could cause restic to not notice
that a file has changed (and therefore continue to store the old version, as
opposed to the modified version) if something edits the file and then resets the
timestamp. Restic now also checks the ctime of files, so any modifications to a
file should be noticed, and the modified file will be backed up. The ctime check
will be disabled if the --ignore-inode flag was given.

If this change causes problems for you, please open an issue, and we can look in
to adding a separate flag to disable just the ctime check.

https://github.com/restic/restic/issues/2179
https://github.com/restic/restic/pull/2212
