tests/test-issue612

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

mkdir t
cd t

hg init
mkdir src
echo a > src/a.c
hg ci -Ama -d "10000000 0"

hg mv src source
hg ci -Ammove -d "1000000 0"

hg co -C 0
echo new > src/a.c
echo compiled > src/a.o
hg ci -mupdate -d "1000000 0"

hg st

hg merge

hg st