Do not touch the original file if the result is identical to the original

[paugier]

Fixes #357

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Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (759826a) compared to base (1493e57). Patch coverage: 100.00% of modified lines in pull request are covered.

:exclamation: Current head 759826a differs from pull request most recent head a9f6ffc. Consider uploading reports for the commit a9f6ffc to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #358   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          815       818    +3     
=========================================
+ Hits           815       818    +3     

Impacted Files	Coverage Δ
src/mdformat/_util.py	100.00% <100.00%> (ø)

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[hukkin]

Thanks, this is great!

I added a couple tests and merged.