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, 2 insertions, 1 deletions

diff --git a/subplot/client.py b/subplot/client.py
index 53c7f6e..d450b4c 100644
--- a/subplot/client.py
+++ b/subplot/client.py
@@ -1,3 +1,4 @@
+import logging
 import os
 import yaml
 
@@ -63,7 +64,7 @@ def capture_generation_id(ctx, varname=None):
     stdout = runcmd_get_stdout(ctx)
     gen_id = "unknown"
     for line in stdout.splitlines():
-        if line.startswith("gen id:"):
+        if line.startswith("generation-id:"):
             gen_id = line.split()[-1]
 
     v = ctx.get("vars", {})