sync, merge, post-receive: Avoid merging unrelated histories, which used to
be allowed only to support direct mode repositories.
(However, sync does still merge unrelated histories when importing trees
from special remotes, and the assistant still merges unrelated histories
always.)
See 556b2ded2b for why this was added
back in 2016, for direct mode.
This is a behavior change, which might break something that was relying
on sync merging unrelated histories, but git had a good reason to
prevent it, since it's easy to foot shoot with it, and git-annex should
follow suit.
Sponsored-by: Noam Kremen on Patreon
b6bea0d3f2