feat: backup run now ends with a summary

"obnam backup" now writes a summary like the following at the end:

status: OK
duration: 24
file-count: 119245
generation-id: ef353c79-a94f-4903-bd80-e741ea454c84

We can add more fields to that later, as needed and requested. This
was the data that's easily at hand.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/backup_progress.rs b/src/backup_progress.rs
index d65f104..6c1d3e6 100644
--- a/src/backup_progress.rs
+++ b/src/backup_progress.rs
@@ -41,5 +41,6 @@ impl BackupProgress {
 
     pub fn finish(&self) {
         self.progress.set_length(self.progress.position());
+        self.progress.finish_and_clear();
     }
 }