Fix issue with mv and certain patterns

[DrChainsaw]

Error (introduced by me in #46) was kind of obvious when looking at the code but was surprisingly difficult to trigger.

[codecov-io]

Codecov Report

Merging #48 (f88746f) into master (53a5aaf) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
  Coverage   86.98%   86.98%           
=======================================
  Files           6        6           
  Lines         361      361           
=======================================
  Hits          314      314           
  Misses         47       47           

Impacted Files	Coverage Δ
src/tree-ops.jl	82.85% <100.00%> (ø)

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