tests/test-nested-repo

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 init b
echo x > b/x

echo '# should print nothing'
hg add b
hg st

echo '# should fail'
hg st b/x
hg add b/x

echo '# should fail'
hg add b b/x
hg st

echo '# should arguably print nothing'
hg st b

echo a > a
hg ci -Ama a

echo '# should fail'
hg mv a b
hg st