rebase: act on command line outside parsing loop

rebase: act on command line outside parsing loop

To later be able to use the command line processing in git-rebase.sh
for both interactive and non-interactive rebases, move anything that
is specific to non-interactive rebase outside of the parsing
loop. Keep only parsing and validation of command line options in the
loop.

Signed-off-by: Martin von Zweigbergk <redacted>
Signed-off-by: Junio C Hamano <redacted>