Hi,

when there's nothing to rebase (ie. upstream..branch is empty), rebase
fails to find any commits to rebase and correctly says "Nothing to do",
but when --onto is given, we already did a "reset --hard" to newbase, so
it already _did_ something.

As rebase also shows a "fatal: Not a range", during the operation, I
assume that this is an actual bug and not just a plain user error.

A test script is attached.

Thank,
Bj