rebase: use 'git stash store' to simplify logic

rebase: use 'git stash store' to simplify logic

rebase has no reason to know about the implementation of the stash.  In
the case when applying the autostash results in conflicts, replace the
relevant code in finish_rebase () to simply call 'git stash store'.

Signed-off-by: Ramkumar Ramachandra <redacted>
Signed-off-by: Junio C Hamano <redacted>