Fix unnamed register bug

[m-pilia]

Ensure that content deleted from the yggdrasil buffer during the
rendering of the tree is discarded and not stored in the unnamed
register. This fixes a bug that causes the content of the unnamed yank
register being discarded when entering an yggdrasil buffer.

Resolves #24

[codecov-commenter]

Codecov Report

Merging #25 (849a11f) into master (56bc96e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #25   +/-   ##
=======================================
  Coverage   98.81%   98.81%           
=======================================
  Files           3        3           
  Lines         254      254           
=======================================
  Hits          251      251           
  Misses          3        3           

Impacted Files	Coverage Δ
autoload/yggdrasil/tree.vim	100.00% <100.00%> (ø)

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