tests/test-empty

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 log
hg grep wah
hg manifest
hg verify
ls .hg
ls .hg/store

cd ..
hg clone a b
cd b
hg verify
ls .hg
ls .hg/store