Remove the parsing and validation of references (onto, upstream, branch)
from git-rebase--interactive.sh and rely on the information exported from
git-rebase.sh.

By using the parsing of the --onto parameter in git-rebase.sh, this
improves the error message when the parameter is invalid.
Signed-off-by: NMartin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>