tests/test-issue619

Add symlink method to util.opener.

Differences from os.symlink:

- the symlink name is relative to the opener base directory

- if a file with that name already exists, it's removed

- if necessary, parent directories are created

- if the system (OS or filesystem) doesn't support symlinks, a
  regular file is created.  Its contents are the symlink target.

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg ci -Ama -d '1000000000 0'
echo b > b
hg branch b
hg ci -Amb -d '1000000000 0'
hg co -C 0

echo fast-forward
hg merge b
hg ci -Ammerge -d '1000000000 0'

echo bogus fast-forward should fail
hg merge b

echo done