git merge has recently been made to refuse to merge disconnected histories
unless --allow-unrelated-histories is passed. This will break uses of the
webapp, eg local pairing until git-annex is changed to pass that whenever
it runs `git merge`.

It could also perhaps break uses of `git annex sync` where a remote with a
disconnected history is added and it's expected to merge with it. Although
in this latter case, it might be argued that the default git behavior has
changed and `git annex sync` should follow suite.

(Also, any uses of `git pull` currently would need to
be split into a fetch and a merge in order to pass the option to the merge;
but AFAICS, git-annex never uses `git pull`)

--[[Joey]]

> [[done]]; used the environment variable
> `GIT_MERGE_ALLOW_UNRELATED_HISTORIES` which will hopefully land in git
> `next` (currently Junio has posted a patch but not even landed it in `pu`
> yet) --[[Joey]]
