shrink-revlog: improve performance: use changegroup instead of revisions
Before:
real 0m23.971s
Now:
real 0m4.229s
The only case where the output would be different is if the newer hg was using
a different diff algorithm than used originally.
adding a/b/c/d/x
adding a/b/c/d/y
adding a/b/c/d/z
moving a/b/c/d/x to z/b/c/d/x
moving a/b/c/d/y to z/b/c/d/y
moving a/b/c/d/z to z/b/c/d/z