One of the issues with correctly configuring a shared mercurial repository is to prevent there being multiple heads in the shared repository. Multiple heads? Does that sound like Zaphod Beeblebrox? Maybe, maybe not. In Mercurial history can be a convoluted thing, just like in real life. The series of changes a team has in hand may end up at a place where there are multiple lines of history. A head revision is one which does not have any children, that is a head revision is one from which no further revisions have been made. There is always at least one head revision, that's the tip. But there may be multiple heads.
For many reasons it is bad practice for a shared repository to have multiple heads. Doing a push from a repository that contains multiple heads also makes the repository into which the push lands also have multiple heads.
One method is the following hgrc configuration
pretxnchangegroup.forbid_2heads = /path/to/forbid_2head.sh
The hook calls this shell script
% cat /path/to/forbid_2head.sh
COUNT=`hg heads | grep "^changeset:" | wc -l`
if [ "$COUNT" -ne "1" ] ; then
echo "Trying to push more than one head, try run "hg merge" before it"