Fix merge key order, clear anchors after resolving

[biojppm]

Fixes #484
Fixes #400

[codecov]

Codecov Report

Attention: Patch coverage is 94.68085% with 5 lines in your changes missing coverage. Please review.

Project coverage is 97.37%. Comparing base (d3132a2) to head (5f1bcee).
Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
src/c4/yml/tree.cpp	92.30%	3 Missing ⚠️
src/c4/yml/reference_resolver.cpp	96.15%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #506      +/-   ##
==========================================
- Coverage   97.37%   97.37%   -0.01%     
==========================================
  Files          34       34              
  Lines       11472    11507      +35     
==========================================
+ Hits        11171    11205      +34     
- Misses        301      302       +1     

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[coveralls]

Pull Request Test Coverage Report for Build 14069134494

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 60 of 60 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 97.578%

---

Totals
Change from base Build 14022406418:	0.05%
Covered Lines:	11564
Relevant Lines:	11851

---

💛 - Coveralls

[biojppm]

The coverage downgrade is likely spurious; merging now.