rebase: read state outside loop

rebase: read state outside loop

The 'onto_name' state used in 'git rebase --merge' is currently read
once for each commit that need to be applied. It doesn't change
between each iteration, however, so it should be moved out of the
loop. This also makes the code more readable. Also remove the unused
variable 'end'.

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