rebase --merge: return control to caller, for housekeeping

Return control to the caller git-rebase.sh to get these two tasks

    rm -fr "$dotest"
    git gc --auto

done by it.

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

git-rebase--merge.sh

@@ -96,7 +96,6 @@ finish_rb_merge () {
 			"$GIT_DIR"/hooks/post-rewrite rebase <"$state_dir"/rewritten
 		fi
 	fi
-	rm -r "$state_dir"
 	say All done.
 }
 
@@ -110,7 +109,7 @@ continue)
 		continue_merge
 	done
 	finish_rb_merge
-	exit
+	return
 	;;
 skip)
 	read_state
@@ -122,7 +121,7 @@ skip)
 		continue_merge
 	done
 	finish_rb_merge
-	exit
+	return
 	;;
 esac