tests/test-rebuildstate

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
# basic test for hg debugrebuildstate

hg init repo
cd repo

touch foo bar
hg ci -Am 'add foo bar'

touch baz
hg add baz
hg rm bar

hg debugrebuildstate
echo '% state dump after'
hg debugstate --nodates | sort
echo '% status'
hg st -A