Merge ignored for node without parent

When node has no parent, the merging of keys does not happen. See the reproducer

```
$ cat <<EOF > main.fmf
k: a
k+: b
EOF
$ fmf show
/
k+: b
k: a
$ mv main.fmf foo.fmf
$ fmf show
/foo
k: ab
```

IMO the very first node should have the merging processed as well.