fix: Avoid re-calculating md5sum on clone and conversion to KmerMinHashBTree

[luizirber]

While debugging sourmash-bio/sourmash_plugin_branchwater#503 the flamegraph showed ~26% of the time was spent on calculating MD5.

WHY????

Turns out cloning and converting to KmerMinHash to KmerMinHashBTree triggered recalculation of the MD5 sum, even if it was already present (or... not needed). Oops!

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.46%. Comparing base (e86c8a8) to head (d488294).
Report is 1 commits behind head on latest.

Files with missing lines	Patch %	Lines
src/core/src/sketch/minhash.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3385      +/-   ##
==========================================
- Coverage   86.47%   86.46%   -0.01%     
==========================================
  Files         137      137              
  Lines       16089    16092       +3     
  Branches     2219     2219              
==========================================
+ Hits        13913    13914       +1     
- Misses       1869     1871       +2     
  Partials      307      307              

Flag	Coverage Δ
hypothesis-py	25.43% <ø> (ø)
python	92.40% <ø> (ø)
rust	62.23% <75.00%> (-0.03%)	⬇️

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[ctb]

nice!