!from_yaml seems to break YAML merge key on mappings

[fortenbt]

Mappings defined in other YAML files cannot be merged using the YAML merge key. When attempting this, the following error occurs.

$ scuba foo
scuba: Config error: Error loading .scuba.yml: while constructing a mapping
  in ".scuba.yml", line 8, column 13
expected a mapping for merging, but found scalar
  in ".scuba.yml", line 3, column 22

Background

YAML mappings can be merged with with merge key << (https://yaml.org/type/merge.html)

Example
The example in this issue uses the following files: foo.yml, bar.yml, and .scuba.yml

# foo.yml
aliases:
    foo:
        global-vars: &global-vars
            VERSION: "1.0"
        sub-vars: &sub-vars
            SUB_VERSION: "2.0"
        environment:
            <<: [ *global-vars, *sub-vars ]

# bar.yml
variables:
    VERSION: "1.0"

subvars:
    SUB_VERSION: "2.0"

# .scuba.yml (just foo.yml, except replacing the *-vars values with !from_yaml)
aliases:
    foo:
        global-vars: &global-vars
            !from_yaml bar.yml variables
        sub-vars: &sub-vars
            !from_yaml bar.yml subvars
        environment:
            <<: [ *global-vars, *sub-vars ]

PyYAML Output

>>> import yaml
>>> yaml.safe_load(open('foo.yml', 'r').read())
{'aliases': {'foo': {'global-vars': {'VERSION': '1.0'}, 'sub-vars': {'SUB_VERSION': '2.0'}, 'environment': {'SUB_VERSION': '2.0', 'VERSION': '1.0'}}}}

Expected Behavior
The environment key contains the merged variables and subvars mappings from bar.yml:

'environment': {'SUB_VERSION': '2.0', 'VERSION': '1.0'}

General Notes

Didn't look into this too hard, but config.py defines from_yaml and uses construct_scalar, which is probably related.

[fortenbt]

Looked into it a bit more. It appears the node type handed to your from_yaml constructor is already a ScalarNode, when I think it should be a MappingNode. Without understanding how PyYAML really intended on doing this, it probably involves using the Resolver, which is how they tag the << merge key node with 'tag:yaml.org,2002:merge' to begin with.

[haboustak]

A similar issue was reported in pyyaml upstream: yaml/pyyaml#120

In reviewing the code, it seems like a merge key mapping does not construct custom tags. If it did call the !from_yaml constructor, then it would have received a mapping value (at least in your example, but not in every example).

I think this would have to be fixed in pyyaml by adding support and examples to the above issue.