Summary: Now trivial move is only triggered when moving from level n to n+1. With dynamic level base, it is possible that file is moved from level 0 to level n, while levels from 1 to n-1 are empty. Extend trivial move to this case.

Test Plan: Add a more unit test of sequential loading. Non-trivial compaction happened without the patch and now doesn't happen.

Reviewers: rven, yhchiang, MarkCallaghan, igor

Reviewed By: igor

Subscribers: leveldb, dhruba, IslamAbdelRahman

Differential Revision: https://reviews.facebook.net/D36669