jj: show author timestamp

instead of commit timestamp

config/jj/config.toml

@@ -47,6 +47,7 @@ concat(
 '''
 
 [template-aliases]
+'commit_timestamp(commit)' = 'commit.author().timestamp()'
 'format_short_signature(signature)' = 'coalesce(signature.email().local(), email_placeholder)'
 log_medium = '''
 if(root,