Fix cold cache diff performance
cold cache diff performance has regressed in two ways. localrepo.changes
has optimizations for diffing against the working dir parent that expect
node1 to be None. commands.revpair() usage means that commands.dodiff()
never sends node1 == None. This is fixed in localrepo.changes by checking
against the dirstate parents.
In the non-dirstate parents case, localrepo.changes does a loop comparing
files without first sorting the file names, leading to random access
across the disk.
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