revlog: drop some unneeded rev.node calls in revdiff

revlog: avoid an expensive string copy This showed up in a statprof profile of "hg svn rebuildmeta", which is read-intensive on the changelog. This two-line patch improved the performance of that command by 10%.

changegroupsubset: avoid setdefault in inner loop

changegroupsubset: drop repeated len() calls

changegroup: reduce progress overhead

changegroupsubset: renest inner loop The mdata list should generally be shorter than changedfiles

revlog: increase readahead size

revset: avoid demandimport bug Apparently the "import x as xy" doesn't manage to update xy in the current scope's dictionary after load, which causes nodemod.nullrev to do a huge amount of demandload magic in the inner loop.