Pretty format json output.

This time correctly, hopyfully.

borgmatic/borg/create.py

@@ -157,4 +157,5 @@ def create_archive(
     )
 
     logger.debug(' '.join(full_command))
-    subprocess.check_call(full_command)
+    output = subprocess.check_output(full_command)
+    return output.decode() if output is not None else None

borgmatic/commands/borgmatic.py

@@ -155,7 +155,7 @@ def _run_commands(args, consistency, local_path, location, remote_path, retentio
             unexpanded_repository,
         )
     if args.json:
-        sys.stdout.write(json.dumps(json_results))
+        sys.stdout.write(json.dumps(json_results, indent=2))
 
 
 def _run_commands_on_repository(
@@ -176,14 +176,19 @@ def _run_commands_on_repository(
         )
     if args.create:
         logger.info('{}: Creating archive{}'.format(repository, dry_run_label))
-        borg_create.create_archive(
+        output = borg_create.create_archive(
             args.dry_run,
             repository,
             location,
             storage,
             local_path=local_path,
             remote_path=remote_path,
+            json=args.json,
         )
+        if args.json:
+            json_results.append(json.loads(output))
+        else:
+            sys.stdout.write(output)
     if args.check:
         logger.info('{}: Running consistency checks'.format(repository))
         borg_check.check_archives(