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.
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
created new head
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
created new head
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
created new head