I left out branches and custom namespaces on purpose from D9252
because I figured that people like us (Google) who have custom
namespaces can also have custom configs. However, I just realized that
this makes everyone with the topic extension lose the topic they've
had in rebase output for a long time (ever since someone was nice
enough to add it in D741). Sorry about the churn.
The more generic template couldn't easily keep the "log.bookmark"
label in the template because the namespace is called "bookmarks"
(plural). That means that we can't be compatible with users' existing
configs for "log.bookmark", so I decided to change the labels to be in
a brand-new "oneline-summary" namespace.