Turns out, it isn't true though the diffs are still empty, i.e.
$ git log --name-status --no-merges b1..b2
produces bogus output when b1 is the a branch from the main hg repo and
b2 a fork of it. But
$ git diff b1..b2
still produces the correct result.
Signed-off-by: Rocco Rutte <pdmef@gmx.net>