revlog: reuse cached delta for identical base revision (issue5975) Since 8f83a953dddf, we skip over empty deltas when choosing a delta base. Such delta happens when two distinct revisions have the same content. The remote might be sending a delta against such revision within the bundle. In that case, the delta base is no longer considered, but the cached one could still, be used with the equivalent revision. Not reusing the delta from the bundle can have a significant performance impact, so we now make sure with doing so when possible.

snapshot: fix line order when skipping over empty deltas The code movement in 37957e07138c introduced an error. Since 8f83a953dddf, we discarded some revisions because they are identical to their delta base (and use that delta base instead). That logic is good, however, in 37957e07138c we mixed up the order of two line, adding the "new" revision to the set of already tested one, instead of the discarded one. So in practice, we were never investigating any revisions in a chain starting with an empty delta. Creating significantly worst delta chain (eg: Mercurial's manifest move goes from about 60MB up to about 80MB).