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
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