tests/test-diff-reverse

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
cat > a <<EOF
a
b
c
EOF
hg ci -Am adda

cat > a <<EOF
d
e
f
EOF
hg ci -m moda

hg diff --reverse -r0 -r1

cat >> a <<EOF
g
h
EOF
hg diff --reverse --nodates