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.
% reverting to rev 0
reverting foo
M foo
diff --git a/foo b/foo
old mode 100755
new mode 100644