[MERGE] Update to Knit._check_should_delta

[John Arbash Meinel]

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According to lsprof, the primary cost to "KnitVersionedFile._add()" is
actually _check_should_delta. (12.5s/23.5s in add_lines()).
This isn't a major factor in the overall time (292s), but it doesn't
hurt to fix it up.

The patch not only fixes it to make a single request of the index rather
than multiple, it also makes it correct when the compression parent is
not the left-hand parent.

(Also, this doesn't show a measurable improvement in wall-clock time for
the first 1k revs without lsprof, however the correctness portion seemed
enough to merge it.)

John
=:->

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