merge: make 'diverging renames' diagnostic a more helpful note.
See the Hg Book on why we actually want to detect this case:
http://hgbook.red-bean.com/read/mercurial-in-daily-use.html#id364290
Before:
$ hg up
deadbeef
warning: detected divergent renames of X to:
...
After:
$ hg up
deadbeef
note: possible conflict - X was renamed multiple times to:
...
No functionality change.