rebase: simplify code related to imply_merge()

rebase: simplify code related to imply_merge()

The code's evolution left in some bits surrounding enum rebase_type that
don't really make sense any more. In particular, it makes no sense to
invoke imply_merge() if the type is already known not to be
REBASE_APPLY, and it makes no sense to assign the type after calling
imply_merge().

enum rebase_type had more values until commit a74b35081c ("rebase: drop
support for `--preserve-merges`") and commit 10cdb9f38a ("rebase: rename
the two primary rebase backends"). The latter commit also renamed
imply_interactive() to imply_merge().

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>