perf: fix `perfhelper-mergecopies` report of #changesets

perf: fix `perfhelper-mergecopies` report of #changesets

Same as `perfhelper-pathcopies`. The previous computation `<base>::<target>` was
wrong, what we actually need is `::<target> - ::<base>`.

This is now fixed.

--- a/contrib/perf.py	Wed Oct 02 18:39:20 2019 -0400
+++ b/contrib/perf.py	Thu Oct 10 04:48:31 2019 +0200
@@ -1941,10 +1941,10 @@
             data = {
                 b'base': b.hex(),
                 b'p1.node': p1.hex(),
-                b'p1.nbrevs': len(repo.revs('%d::%d', b.rev(), p1.rev())),
+                b'p1.nbrevs': len(repo.revs('only(%d, %d)', p1.rev(), b.rev())),
                 b'p1.nbmissingfiles': len(p1missing),
                 b'p2.node': p2.hex(),
-                b'p2.nbrevs': len(repo.revs('%d::%d', b.rev(), p2.rev())),
+                b'p2.nbrevs': len(repo.revs('only(%d, %d)', p2.rev(), b.rev())),
                 b'p2.nbmissingfiles': len(p2missing),
             }
             if dostats: