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 <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -153,11 +153,8 @@ finish_rebase () {
 		then
 			echo "$(gettext 'Applied autostash.')"
 		else
-			ref_stash=refs/stash &&
-			>>"$GIT_DIR/logs/$ref_stash" &&
-			git update-ref -m "autostash" $ref_stash $stash_sha1 ||
-			die "$(eval_gettext 'Cannot store $stash_sha1')"
-
+			git stash store -m "autostash" -q $stash_sha1 ||
+			die "$(eval_gettext "Cannot store \$stash_sha1")"
 			gettext 'Applying autostash resulted in conflicts.
 Your changes are safe in the stash.
 You can run "git stash pop" or "git stash drop" it at any time.