tests/test-issue436

prepush: add more precise error messages

Part of the patch is from [e-mail]

- 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.

#!/bin/sh

hg init a
cd a
hg -v log -v
hg -v log -v x
true