1-click ops: improve performance of "back up apps with changes"

Calculating hash values for each directory can be very time consuming. Instead,
a greedy approach has been adopted that looks up to depth 3 recursively to find
changes in the timestamp (access and/or modification time). While this approach
may miss a few cases, it's less time consuming that the previous method.

Signed-off-by: Muntashir Al-Islam <muntashirakon@riseup.net>

Author: MuntashirAkon