rebase -i: use struct commit when parsing options

rebase -i: use struct commit when parsing options

This is in preparation for using `struct rebase_options` when parsing
options in cmd_rebase__interactive(). Using a string for onto,
restrict_revision and upstream, was a hangover from the scripted version
of rebase. The functions that use these variables are updated to take a
`struct commit`.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>