Bug#909023: works between two 6.20180913-1 repos

[Marco]

I tried with two clean repositories, one bare and one
non-bare. Initially the bare one was from stretch (6.20170101-1+deb9u2)
and the other from testing (6.20180913-1), the issue sowed up. Then I
upgraded the bare one from stretch/backports (6.20180913-1~bpo9+1)
aaaand... it worked!

So I guess this is just a backward compatibility issue, would be nice to
know if there's any chance to fix it or else I'm going to move my
central repo on some other machine on which I can keep git-annex more up
to date.

thanks
m