etcupdate failed to build new tree

[Graham Perrin]

At what should have been the end of my first upgrade since the 
transition to git:

cd /usr/src/freebsd-current && make installworld && etcupdate

𠄴– concluded with a successful installworld, then a few lines about 
scanning and skipping certificates, then:

 > Failed to build new tree.

I see the manual page for etcupdate(8) but I'm not sure how to proceed.

----

I did try `mergemaster -p` and somehow (!) ended up with an empty 
`/etc/group` file. Dug myself out of that hole, I'll prefer to continue 
using etcupdate.