Am Do., 31. Okt. 2019 um 23:53 Uhr schrieb Jonathan Nieder [off-list ref]:

I wonder what this warning is trying to say.  How would I go about
triggering this message?  The comment before says

        The relevant upstream is the fetched branch that is meant to
        be merged with the current one, i.e. the one fetched to
        FETCH_HEAD.

So is this about when I'm fetching with a wildcard or something?

The message shows up when multiple refspecs/branches are passed,
like

$ git fetch --set-upstream origin master next
From https://github.com/git/git
 * branch                  master     -> FETCH_HEAD
 * branch                  next       -> FETCH_HEAD
warning: multiple branch detected, incompatible with --set-upstream