keyword: copy: when copied source is a symlink, follow it
1) hg cp symlink copy -> copy is a symlink.
2) cp symlink copy; hg cp -A symlink copy -> copy is a regular file.
In the second case we have to follow the symlink to its target
to find out whether we have to unexpand keywords in the copy.
Add test covering the case where the copied link's target is ignored
by keyword but has content which would match the regex for expanded
keywords to check whether we indeed leave the destination alone.