sparse-revlog: only refine delta candidates in the sparse case (issue6006) Starting with 5aef5afa8654, a valid delta parent might be "refined". This allows repository using sparse-revlog to produce better delta chain by using better intermediate snapshot base. However, this refining step was performed in all cases, including for repository not using sparse-revlog. This could produce a strange chain in the general delta case and corrupted repository in the non-general delta case. We now skip this step unless sparse-revlog is in use. In issue 6006, Yuya Nishihara provided a test case using an external repository, so we did not include it. Finding "laboratory" condition to reproduce this case and implementing an efficient test reproducing it is a bit tricky. We do not foresee to have the time to provide one by the release date. Differential Revision: https://phab.mercurial-scm.org/D5197