rebase: report invalid commit correctly

rebase: report invalid commit correctly

In 9765b6a (rebase: align variable content, 2011-02-06), the code
to error out was moved up one level. Unfortunately, one reference
to a function parameter wasn't rewritten as it should, leading to
the wrong parameter being errored on.

This error was propagated by 71786f5 (rebase: factor out reference
parsing, 2011-02-06) and merged in 78c6e0f (Merge branch
'mz/rebase', 2011-04-28).

Correct this by reporting $onto_name istead.

Reported-By: Manuela Hutter <manuelah@opera.com>
Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>