prepush: add more precise error messages
Part of the patch is from timeless@mozdev.org
- indicate the branch name where there are multiple heads
- give better advice when hitting a possible race, where new heads are added
between discovery and the call to branchmap(). In that case, asking the user
to merge isn't helpful, since only remote has the changes.
% change the id of the tree root
initializing destination source-hg repository
scanning source...
sorting...
converting...
1 Initial add
0 Changed root
% manifest of tip
644 file