Fix backup directory format, it now ends with '.previous'

[antimirov]

Hi guys,

This fixes the #17 (and probably #18 too). At least I hope it fixes it because I only downloaded your tool yesterday and as I have Big Sur, it just showed me the message "None or only one Time Machine backups found at /usr/local/bin/timedog line 197."

My backups directory structure is the following:

2021-03-12-010326.interrupted <-- skipping these ".interrupted", not sure what's there
2021-03-12-174648.previous
2021-03-13-110439.interrupted  <-- skipping these ".interrupted", not sure what's there
2021-03-15-154701.previous
2021-03-15-175421.previous

With this simple fix I'm now able to see the diffs.

[nlfiedler]

Excellent, thank you.