rebase -i: combine rebase--interactive.c with rebase.c

rebase -i: combine rebase--interactive.c with rebase.c

In order to run `rebase -i` without forking `rebase--interactive` it
will be convenient to have all the code from rebase--interactive.c in
rebase.c. This is a straight forward copy of the code from
rebase--interactive.c, it will be simplified slightly in the next
commit.

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