Bugfix: Correct `find` to not skip snapshots

Under certain circumstances, the `find` command was found to skip snapshots
containing directories with files to look for when the directories haven't been
modified at all, and were already printed as part of a different snapshot. This
is now corrected.

In addition, we've switched to our own matching/pattern implementation, so now
things like `restic find "/home/user/foo/**/main.go"` are possible.

https://github.com/restic/restic/issues/1825
https://github.com/restic/restic/issues/1823