yaml-merge ignores anchors=left when aliased hash merge keys are present

[wwkimball]

Operating System

1. Name/Distribution: ANY
2. Version: ANY

Version of Python and packages in use at the time of the issue.

1. Distribution: ANY supported
2. Python Version: ANY supported
3. Version of yamlpath installed: 3.6.4
4. Version of ruamel.yaml installed: ANY supported

Minimum sample of YAML (or compatible) data necessary to trigger the issue

---
# lhs.yaml
aliases:
  - &name_a a_name
  - &name_b b_name

ref_hash: &ref_hash
  *name_a : {}
  *name_b : {}

imp_hash_a:
  <<: *ref_hash

imp_hash_b:
  <<: *ref_hash

---
# rhs.yaml
aliases:
  - &name_a NEW_A_VALUE
  - &name_b NEW_B_VALUE

Complete steps to reproduce the issue when triggered via:

1. yaml-merge --anchors=left lhs.yaml rhs.yaml

Expected Outcome

---
# lhs.yaml
aliases:
  - &name_a NEW_A_VALUE
  - &name_b NEW_B_VALUE

ref_hash: &ref_hash
  *name_a : {}
  *name_b : {}

imp_hash_a:
  <<: *ref_hash

imp_hash_b:
  <<: *ref_hash

Actual Outcome

---
# lhs.yaml
aliases:
  - &name_a a_name
  - &name_b b_name

ref_hash:
  *name_a : {}
  *name_b : {}

imp_hash_a:
  anchor01: &ref_hash
    NEW_A_VALUE : {}
    NEW_B_VALUE : {}

imp_hash_b:
  <<: *anchor01
  NEW_A_VALUE : {}
  NEW_B_VALUE : {}

Note that the ref_hash anchor moves and gets redefined where the first reference formerly existed, then ignored for the second reference.

[This is from memory and the detail of the actual outcome may be slightly different from that hand-typed above. The point is, the merge causes a a redefinition of the original anchor and an "explosion" of its aliases.]

[wwkimball]

This issue was fixed along with #191.