evolve: show a status message when merging leads to public cset
I think it would be fine to tell the user that merging the two divergent
csets while resolving public divergence resulted into the public cset
itself.
If you think we don't need to tell the user, then we can include this
in debug-mode for debugging purposes at least.
Test that share-clones use the cache in the original repository.
$ . $TESTDIR/testlib/common.sh
$ hg init share-base
$ cd share-base/
$ cat >> .hg/hgrc <<EOF
> [extensions]
> evolve=
> share=
> EOF
$ hg debugbuilddag .+3:branchpoint+4*branchpoint+2
$ cd ..
$ hg --config extensions.share= share -U share-base share-client
$ cd share-client
$ hg debugupdatecache
$ test -d .hg/cache
[1]
$ test -d ../share-base/.hg/cache